Antisperm antibodies (ASA) appear to impair reproduction; however, their clinical significance in in vitro fertilization-embryo transfer (IVF-ET) is unestablished. For examination of this question, the immunobead binding technique was used to identify IgA, IgG, and IgM ASA in the serum, semen, and follicular fluid (FF) of 40 couples undergoing IVF-ET. ASA binding to sperm tail tip did not predict the fertilization rate of uniformly inseminated mature oocytes. Similarly, ASA binding to sperm head in semen and male serum did not predict fertilization. However, the fertilization rate in couples with ASA to sperm head (ASA-H) of at least one isotype in female serum (n = 6) was significantly less than in those without ASA-H (n = 34; 34% versus 74%, P less than 0.01). Among these women, oocyte fertilization rates were 33% versus 71% (P less than 0.001). Sixty percent of women whose ova did not fertilize (n = 5) had ASA-H in their serum versus 6% of those whose ova did (n = 35; P less than 0.05). The presence of ASA-H in FF also correlated with fertilization. ASA-H in female serum reduced the zygote cleavage rate from 91% to 67% (P = 0.51). We conclude that the presence of ASA-H in female serum and FF is associated with reduced fertilization in IVF-ET.