Follicular fluid and progesterone, which are present in the natural environment of oocytes, have been reported to induce the acrosome reaction and we compared their use in pretreatment of spermatozoa for human sub-zonal insemination (SUZI). Pre-treatment with follicular fluid (20% v/v) was associated with a higher fertilization rate than pre-incubation with progesterone (1 mmol/l) as assessed by both the embryos/injected oocytes rate (31.7 +/- 6.2% versus 13.5 +/- 5.9%, respectively; P < 0.01) and the male pronuclei/injected spermatozoon rate (10.5 +/- 3.3% versus 3.6 +/- 1.9%, respectively; P < 0.01). Since we have previously reported that pre-treatment with progesterone allowed a higher percentage of live-reacted spermatozoa to be obtained, these results suggest that either progesterone induces modifications of the plasma membrane, which prevent fusion with the oolema, or that follicular fluid not only induces the acrosome reaction but increases the fusion ability by compounds other than progesterone.