Capacitation of spermatozoa is essential for fertilization, and can be induced by various agents or biological fluids. Previous reports have shown that foetal cord serum (FCS) and the superoxide anion trigger human sperm hyperactivation and capacitation, and that superoxide dismutase (SOD) prevents these processes. We investigated: (1) the capacity of seminal plasma (SP) and follicular fluid (FF)(whole, or fractionated into high and low molecular weight components), in the presence or absence of SOD, to induce the spontaneous acrosome reaction(no stimulant needed, AR) and capacitation (as measured by the lysophosphatidyl-choline-induced AR, LPC-AR); (2) a possible relationship between the levels of AR and capacitation obtained with these biological fluids and the superoxide scavenging capacity of the same fluids. The highest levels of LPC-AR were obtained with FF ultrafiltrate (48 ± 6%), followed by SPultrafiltrate (31.9 ± 0.8%), FF (30 ± 5%), dialysed FF (27 ± 4%), and finally, by FCSultrafiltrate (23 ± 1%.), SP (21 ± 1%) and dialysed SP (18.9 ± 0.8%).A similar order of potency for the fluids existed when sperm AR was studied, the levels of AR observed ranging from 26 ± 2% to 5.3 ± 0.8% after incubation with FF ultrafiltrate and SP respectively. None of these treatments had detrimental effects on sperm motility. In the presence of SOD, there was always an important reduction (52–86%) of the AR and LPC-AR observed. A highly significant inverse linear relationship was observed between the SOD-like activity of the fluids tested and the AR (r= 0.86, p < 0 0.001) and LPC-AR (r= 0.91, p < 0.001)observed in the presence of these fluids. The results suggest that biological fluids contain inducers of the AR and capacitation, and that these probably act by a common mechanism, possibly by direct or indirect induction of an NADPH oxidase in the sperm membrane. The data suggest strongly that the SOD-like activity of a specific fluid is probablyone of the most important factors that will determine its capacity to induce the AR and capacitation.