The effect of sperm concentration on the fertilization of preovulatory and immature human eggs was studied in the context of an ongoing in vitro fertilization-embryo transfer (IVF-ET) program. Fertilization success was independent of the follicular recruitment protocol used, and with preovulatory eggs, was inversely related to sperm concentration over the range of 2.5 - 50 X 10(4) motile sperm/ml. Maximum fertilization (80.8%) occurred at a concentration of 2.5 X 10(4) motile sperm/ml. The incidence of polyspermic fertilization was directly related to the sperm concentration, decreasing from 5.5% at 10 X 10(4) to 0% at 1-2.5 X 10(4) motile sperm/ml. Immature eggs cultured in vitro, then inseminated, also demonstrated an inverse relationship between fertilization and sperm concentration with a maximum fertilization rate of 66.6% at 5 X 10(4) motile sperm/ml. The percentage of motile sperm in the inseminating population had no influence on fertilization rates unless the value dropped below 40%. Fertilization success using sperm from oligospermic and polyzoospermic males was also examined. In contrast to males with normal semen parameters, oligospermic males demonstrated highest fertilization success at 50 X 10(4) motile sperm/ml. The IVF of preovulatory eggs using sperm from polyzoospermic males was comparable to that for males with normal semen parameters at equivalent sperm concentrations. The implications of these findings to the application of IVF-ET technology to the infertile couple is discussed.