The morphological characteristics of mouse thymocytes stained withfluorescein-concanavalin A(F-Con A)and conditions required for the receptorcap formation on thymocytes induced by F-Con A were investigated.Afterincubation of the thymocytes with optimal amount of F-con A,the cells showedbright yellowish green fluorescence under fluorescence microscope.Four types:i.e.evenly stained,patched,deviated and cap types could be observed.The percentageof cap formation was 9.3±3.9%.Of different concentrations of F-Con A,100/μg/ml was the most optimal.Different incubation times(10,20,30 and 40 min)and temperatures did not affect the rate of cap formation.The geomorphological characteristics of the normal mouse thymocytes and theF-ConA-induced cells,as well as the kinetics of capping were investigated bymeans of scanning electronmicroscopy.The normal mouse thymocytes werespherical in shape and had a smooth surface with wavy microvilli,among them,some microvillus-like protrusions interposed.After incubation with F-Con A,the thymocytes could assume three types:i.e.microaggregated,lump-aggregatedand cap types.These three types might correspond to those types observed underfluorescence microscope.The peak of cap formation was at 10 min after beginningof the incubation with F-Con A(30.3%),but at 2 min the cap percentage wasalready 16.9%.This indicated that the rate of cap formation was fast.After thethymocytes were treated with colchicine,the percentage of cap formation wassignificantly increased and the peak(50%)was reached at 2 min.It wassuggested that the destruction of rnicrotubules might promote the cap-formingspeed and thus increase the percentage of cap formation.The cap formationtended to go down as time went on after the thymocytes were treated withcolchicine and were incubated with F-Con A for more than 2 min.The cause ofthis phenomenon awaits further investigation. The morphological characteristics of mouse thymocytes stained with fluid cin-concanavalin A (F-Con A) and conditions required for the receptor cap formation on thymocytes induced by F-Con A were investigated. Afterincubation of the thymocytes with an optimal amount of F-con A, the cells showed bright yellowish green fluorescence under fluorescence microscope. Four types: ieevenly stained, patched, deviated and cap types could be observed. The percentage of cap formation was 9.3 ± 3.9%. Different concentrations of F-Con A, 100 / μg / ml was the most optimal. Different incubation times (10, 20, 30 and 40 min) and temperatures did not affect the rate of cap formation. The geomorphological characteristics of the normal mouse thymocytes and the F-ConA-induced cells, as well as the kinetics of capping were investigated bymeans of scanning electronmicroscopy.The normal mouse thymocytes werespherical in shape and had a smooth surface with wavy microvilli, among them, some microvillus-like protrusions .After incubation with F-Con A, the thymocytes could assume three types: ie microroaggregated, lump-aggregated and cap types. These three types might correspond to those types observed under fluorescence microscope. The peak of cap formation was at 10 min after beginning of the incubation with F This indication that the rate of cap formation was fast. After the thymocytes were treated with colchicine, the percentage of cap formation wassignificantly increased and the peak (50% ) was reached at 2 min.It wassuggested that the destruction of rnicrotubules might promote the cap-formingspeed and thus increase the percentage of cap formation. The cap formationtended to go down as time went on after the thymocytes were treated withcolchicine and were incubated with F -Con A for more than 2 min. The cause ofthis phenomenon awaits further investigation.