AbstractWhen guinea pig spermatozoa are preincubated for 1 hr in Ca2+−free medium containing a low concentration of lysolecithin (LC, 85 μg/ml) and then exposed to 2 mM Ca2+by diluting the preincubation medium with an equal volume of LC−free, 4 mM Ca2+−containing medium, the majority of the spermatozoa undergo acrosome reaction promptly. On the other hand, when the preincubated spermatozoa are exposed to 2 mM Ca2+without reducing the original concentration of LC in the medium, none of them undergo acrosome reaction. These spermatoza can acrosome−react if they are transferred to an LC−free medium. These results and those of some other experiments suggest that in the presistent presence of a high concentration of LC in the medium, exogenous Ca2+essential for the acrosome reaction either does not penetrate the sperm plasma membrane or, if it does, it cannot alter the membrane for the acrosome reaction, at least under the experimental conditions employed. Freeze−fracture examination of the sperm plasma membrane has revealed that small areas or patches free of intramembranous paarticles (IMPs) appear in the membrance during sperm preincubation, and these IMP−free areas expand drastically in response to Ca2+when the LC conccentration in the medium is reduced at the time Ca2+is added to the medium. In contrast, IMP−free areas remain unchanged even after exposure of spermatozoa to Ca2+if the concentration of LC remains at its original l