Androgens are important factors in spermatogenesis. However, the biological role of androgen in Sertoli cells is still unclear. In this study, we analysed mutations and CAG repeats of the androgen receptor gene in 19 azoospermic and 117 oligozoospermic men. No mutations were identified, but 9 of 117 oligozoospermic men were found to carry 14 or 15 CAG repeats, as compared to more than 16 CAG repeats in 136 normal fertile men analysed. Analysis of the androgenic effect on the expression of transition protein 1 and 2 (TP1, TP2) by co-transfection experiments showed that androgens regulate the transcription of TP1 via a Dfd-like molecule and the transcription of TP2 via an androgen-androgen receptor complex, respectively. We analysed the effect of dihydrotestosterone (DHT) on protein profiles in the TM4 mouse Sertoli cell line using SELDI-TOF mass spectrometry. DHT increased the expression of seven proteins of 4.34, 4.97, 5.68, 5.75, 9.95, 9.98 and 11.30 kDa and decreased six proteins of 4.94, 4.97, 6.29, 8.57, 12.39 and 19.81 kDa. One of the decreased molecule (19.80 kDa) is identified to be a translationally controlled tumor protein. These results show that androgen affects spermatogenesis by actions on both the germ cells and Sertoli cells.