Cross-linked complexes formed between certain lectins and their specific multivalent carbohydrates and glycoconjugates on the sperm surface were studied for their ability to modify sperm membrane permeability and to induce the acrosome reaction. Wheat germ agglutinin (WGA), concanavalin A (Con A) and peanut agglutinin (PNA) increased the proportions of human spermatozoa permeable to the impermeable propidium iodide (31.9 compared with 13.8%, 38.4 compared with 18.4% and 72.7 compared with 18.9% respectively). Removal of sperm surface sialic acid by neuraminidase treatment was a prerequisite for Con A and PNA binding to the sperm surface. The percentage of permeable and acrosome-reacted spermatozoa was not affected by sperm treatment with 500 mIU/ml Arthrobacter ureafaciens neuraminidase. WGA did not induce the acrosome reaction, whereas PNA induced the acrosome reaction regardless of the sperm capacitation status, allowing the proportion of acrosome-reacted spermatozoa to reach 27.7% of capacitated spermatozoa. However, the ability of Con A to induce the acrosome reaction was limited to uncapacitated spermatozoa. To test the physiological relevance of this study, uncapacitated human spermatozoa were incubated with human zonae pellucidae and the permeability of spermatozoa bound to the zona surface was analysed according to the time post-insemination. Two-thirds of spermatozoa bound to zona pellucida became permeable to propidium iodide in the first 30 min post-insemination and almost all bound spermatozoa became permeable to the impermeable dye after 60 min. Our results show that molecular interactions between human zona pellucida and sperm surface increase the permeability of sperm membranes; the cross-linked complexes formed by PNA lectin and its specific multivalent carbohydrates and glycoconjugates on the sperm surface were also able to increase sperm membrane permeability and to induce the acrosome reaction. These results suggest a role for the saccharide moieties of sperm surface glycoconjugates in the induction of the acrosome reaction.