We have studied changes in the binding of fluoresceinated lectins to human sperm during in vitro capacitation. We first determined the surface labeling pattern of viable sperm obtained by the swim-up procedure. Sperm were labeled with 100 micrograms/ml FITC-conjugated lectin at 4 degrees C for 30 min. We simultaneously used Hoechst stain 33258 as a supravital stain to help differentiate surface from intracellular lectin labeling. Of 14 lectins studied, six (phytohemagglutinin-E, concanavalin A, Ricinus communis agglutinin-I, and the lectins of wheat germ, Lens culinaris, and Pisum sativum) bound to the entire surface of sperm, sometimes with minor local heterogeneity. Three lectins (from peanut, Maclura pomifera, and soybean) usually bound in a punctate manner, with more label on the tail than on the head. Five lectins (Ulex europaeus, Dolichos biflorus, Helix pomatia, and Vicia villosa lectins, and lectin II of Griffonia simplicifolia) bound very poorly or not at all to the sperm surface. Sperm were also inspected for changes in surface lectin binding patterns after 0, 5, and 23 hr of incubation in a capacitating medium. Two lectins showed reproducible changes. The labeling by Maclura pomifera agglutinin decreased by 5 hr in eight of ten experiments, and among sperm labeled with concanavalin A, the incidence of sperm with a highly fluorescent anterior margin of the sperm head increased by about 3.5-fold between 0 and 5 hr. The labeling pattern of the other lectins did not change.