Serum amyloid P component(SAP)is present in seminal plasma,on spermatozoa,and in different tissues of the male reproductive tract,but its function is not known.The aims of this study were to determine if the concentration of SAP in seminal plasma is associated with commonly assessed semen parameters and to investigate if SAP could be a new,indirect biomarker for these parameters.In a cross-sectional study of 203 young volunteers,the concentration of SAP in seminal plasma was measured with a in-house developed enzyme-linked immunosorbent assay.Scatter plots,Pearson\'s correlation coefficients(r),and linear regression models were produced,and SAP showed a statistically significant correlation with sperm concentration(r=0.75),sperm number(r=0.68),semen volume(r=-0.19),progressive sperm motility(r=0.24),and sperm immotility(r=-0.20).When the study group was dichotomized,SAP could be used to discriminate samples with a sperm concentration＜or≥5×106 ml-1,15×106 ml-1,or 40×106 ml-1,and in receiver operating characteristic curves,the corresponding areas under the curves were 0.97,0.93,and 0.82,respectively,with P＜0.001 for all three cutoff values studied.The concentration of SAP in seminal plasma showed a strong,positive correlation with the concentration of spermatozoa in semen.SAP may be used as a new indirect potential biomarker for sperm concentration in fresh and in frozen,stored samples.In addition,it is envisaged that the assay could be developed into a home fertility test to differentiate between a low and a normal sperm concentration.