Efficient capacitation of hamster spermatozoa can be induced by follicular fluid (Barros & Austin, 1967; Yanagimachi, 1969a, b) and blood sera of several species (Barros & Garavagno, 1970; Yanagimachi, 1970a; Talbot, Franklin & Fussell, 1974). Two components are involved: one is dialysable, heat-stable and stimulates sperm motility, while the other is non-dialysable, heat-labile and induces the acrosome reaction (Yanagimachi, 1969a). Our understanding of capacitation and the acrosome reaction might be enhanced if the nature and mode of action of these factors were known. Blood serum is an easily available source of both factors and this preliminary report describes the recovery of the sperm motility-stimulating activity from human serum. Fresh human serum was heated to 56\s=deg\Cfor 30 min to destroy unidentified toxic factor(s) (Yanagimachi, 1970a). The serum was chromatographed on Sephadex G-25 (medium grade) and equilibrated in Tyrode's solution supplemented with 0\m=.\33mm-sodium pyruvate, 0\m=.\01 mg phenol red/ml, 100 i.u. penicillin/ml and 10 mg Dextran T70 (Pharmacia)/ml. Equilibration in culture medium avoided further dilution of the fractions in the test system. Protein elution was monitored with a L.K.B. Uvicord II measuring absorption at 280 nm and columns were pumped under positive pressure at a flow rate of about 50 ml/cm2/hr. Column calibration was carried out with Blue Dextran (Pharmacia) and potassium ferricyanide. The fraction size was approximately 5% bed volume. In order to test the fractions, 200-μ1 aliquots were placed under paraffin oil in plastic tissue culture dishes (Falcon Plastics) and equilibrated with 5% C02 in air at 37°C. Hamster epididymal spermatozoa were added to the drops so that the final sperm concentration was about 1 106/ml, and the drops were then reincubated. At intervals between 1 and 6 hr, the percentage of motile spermatozoa and the degree of sperm motility in each drop were estimated, and a 'sperm motility index' was calculated as previously described (Bavister, 1974). The proportion of motile spermatozoa showing the acrosome reaction was also noted at 3 and 6 hr. The 'sperm motility-stimulating activity' was constantly found to elute