Spermatogenesis and steroidogenesis are 2 important functions of the testis. It is well known that the 2 pituitary hormones, namely FSH and LH, regulate these functions of the testis. Based on the early work of Greep et al. 1936; Greep&Fevold 1937), FSH is held to be responsible for control of spermatogenesis and LH for testosterone production. However, the precise role of FSH in spermatogenesis is not clear. We used specific and potent antisera to FSH to neutralize this hormone and study its role in spermatigenic function of the immature and adult rat (Madhwa&Dym 1976; Madhwa Raj et al. 1980). Our earlier work on the rat demonstrated the need for FSH during the first wave of spermatogenesis, but not in the adult rat. Further, the precise role of FSH in spermatogenesis of primates including the human male is also not clear. In view of this, we have undertaken studies to examine the effects of neutralizing FSH activity on spermatogenesis in the monkey, M fasicularies.