['To determine the effect of the fusion of prostasomes to spermatozoa on the acrosome reaction.', 'In vitro study of human spermatozoa.', 'Healthy volunteers in an academic research environment.', 'Healthy volunteer men, 25 to 35 years old.', 'Human semen was fractionated into spermatozoa and prostasomes. Fusion of prostasome to spermatozoa was performed at pH 5.5. Progesterone (1 microM) was added when required.', 'Evaluation of the acrosome reaction by fluorescence microscopy.', 'The percentage of spontaneously acrosome-reacted cells was very low unless the Ca(2+)-ionophore A 23187 was added. The treatment of spermatozoa with 1 microM of progesterone scarcely affected the acrosome reaction; a pretreatment in conditions permitting fusion increased it. The addition of progesterone to prostasome-fused spermatozoa further increased the extent of the acrosome reaction.'] Does fusion of prostasomes to human spermatozoa stimulate the acrosome reaction?