ABSTRACT: The effect of sperm antibodies on the human fertilization process was evaluated by analyzing the Royal Women's Hospital in vitro fertilization (IVF) data. The results suggest that sperm autoantibodies, particularly those of IgA immunoglobulin class (determined by immunobead test (IBT)) can interfere with IVF. Thus, in a group (n = 8) of couples where the male partner had 80% or more of his motile spermatozoa coated with IgG and IgA class sperm antibodies, an overall fertilization rate of only 27% (18/66 ova) was obtained. In contrast, in a group (n = 9) with positive IBT results, but with less than 80% of motile spermatozoa coated with IgA class antibodies, a normal fertilization rate of 72% (47/65 ova) was obtained. Three of these patients had 90% or more of their motile spermatozoa coated with IgG and less than 70% coated with IgA class antibodies. In this subgroup a good fertilization rate of 16/21 (76%) was obtained. Another observation derived from this investigation is that the oocytes that do fertilize in the presence of sperm antibodies can subsequently proceed with normal cleavage, implantation, and pregnancy. This provides a rationale for attempting to treat these patients by reducing the proportion of antibody-coated sperm in vitro for future IVF cycles.