In vitro studies were performed to investigate the accessibility of acrosin to various proteinase inhibitors inside the intact acrosome of testicular, ejaculated, and uterine human spermatozoa. As test system the gelatin plate assay was used. For this assay it was shown formerly that a correlation exists between the size of the digested lysis areas (halo formation) and acrosin activity estimated with synthetic substrates. In addition, saturation of the gelatin substrate membranes with acrosin inhibitors including highly specific ones before application of spermatozoa completely prevented halo formation indicating that the gelatinolytic activity of human spermatozoa is caused exclusively by acrosin. When human spermatozoa were incubated with various acrosin inhibitors (concentration: 1 mmol/1) prior to application to the gelatine membrane, reduction of halo formation could not be observed, however. This result indicates that most of the tested acrosin inhibitors (9 naturally occurring protein inhibitors, 2 microbial peptide inhibitors, 19 synthetic inhibitors) were unable to penetrate the acrosomal membr:ines of rrsticular, ejaculated. and uterine human spermatozoa. Only 2 inhibitors caused moderate up to complete inhibition of the gelatinolytic activity of the spermatozoa if applied in concentrations between I and 10 mmul/l: [he proteinase inhibitor aprotinin and the synthetic inhibitor NPCR (4‐nitrophenyl 4‐˜uanidini, benzoate). Obviously, human acrosomal membranes seem to be especially impenetrable to proteins, polypeptides. and synthetic agents. Those acrosin inhibitors penetrating the human sperm head membranes are either too toxic or the local concentration necessary for effectice ncrosin inhihition in vivo cannot be achieved within the male or female genital tract secrelions. Therefore. arrosin inhibitors cannot be used for human contraceptioti at present. Thus it is mandatory to continue the search for suitable acrosin inhibitors with low toxicity easily penetrating into the intact sperm acros