This study characterizes factors present in uterine fluid from oestrous buffaloes which induce the acrosome reaction in buffalo spermatozoa. Characterization was performed by dialysis, heat treatment (90 degrees C, 30 min) and deproteinization of uterine fluid. The % motility and viability of sperm was maintained better in unfractionated uterine fluid than in dialysed fluid. Heating and deproteinization of the dialysed uterine fluid significantly reduced the % motility and viability of sperm. The percentage of sperm showing different stages of the acrosome reaction (swelling, and vesiculation) was significantly higher for sperm incubated in dialysed than in neat uterine fluid. There was no significant difference in the % of sperm showing acrosome shedding in both dialysed and neat uterine fluid. Heat treatment destroyed the ability of uterine fluid to induce the acrosome reaction. Sperm incubated in the protein precipitate from uterine fluid showed all stages of the acrosome reaction. Thus, the uterine factor(s) responsible for inducing the acrosome reaction in buffalo sperm was associated with the non-dialysable, albumin-like protein fraction and was heat-labile.