Both thymocytes and tumor cells express M2 type isoenzyme of pyruvate kinase (M2PK), which is different from R type isoenzyme of pyruvate kinase (RPK) that is expressed in erythrocytes. In this report, the effect of RPK and M2PK on the transcription of human immuno-deficiency virus type 1 (HIV-1) was tested. The results indicated that M2PK could enhance HIV-1 transcription from its long terminal repeat (LTR) promoter, while RPK did not have such an effect. Specific down-regulation of M2PK could inhibit HIV-1 transcription from its LTR region. Furthermore, it was found that the C terminal region of M2PK is responsible for this effect. Collectively,the cellular factor M2PK that is expressed in thymocytes could facilitate the transcription of HIV-1.