We have identified an estrogen receptor of approximately 29 kDa apparent molecular weight in human sperm membranes by ligand and Western blot analysis, respectively, using peroxidase-conjugated E-2 and an antibody directed against the ligand binding region of the genomic receptor (alpha H222). Such receptor is functional since 17 beta E-2 induces a rapid and sustained increase of intracellular calcium concentrations ([Ca2+](i)) which is completely eliminated by preincubation with alpha H222. 17 beta E-2 effects on calcium are clearly mediated by a membrane receptor, as they are reproduced by the membrane-impermeable conjugate of the hormone BSA-E-2. Dose-response curve for this effect is biphasic with EC(50)s in the nanomolar and micromolar range. In addition to calcium increase, 17 beta E-2 stimulates tyrosine phosphorylation of several sperm proteins including the 29-kDa protein band. Preincubation of human sperm with 17 beta E-2 inhibits calcium and acrosome reaction increases in response to progesterone. We conclude that estrogens may play a role in the modulation of non-genomic action of progesterone (P) in human sperm during the process of fertilization. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.