Couples undergoing in vitro fertilization treatment (IVF) were invited to take part in a controlled prospective clinical trial. The aim was to determine the effect on the fertilization rate of a technique devised to obtain an antibody-free preparation of spermatozoa from an antibody-positive ejaculate. Oocytes collected during IVF were allocated into one of two groups, ensuring that quality and maturity were comparable in each. One group, the control, was inseminated with Percoll-processed spermatozoa. The experimental group was inseminated with identical numbers of Percoll-processed spermatozoa which had been treated to obtain an antibody-free preparation. The treatment was found to have no beneficial effect on the fertilization rate at IVF. Laboratory studies were also performed on the ejaculates of antibody-positive volunteers to determine whether this treatment led to any effects, whether beneficial or detrimental, on sperm function. Membrane integrity was found to be unaffected, as was the percentage of spermatozoa undergoing the spontaneous acrosome reaction following overnight incubation. The percentage of spermatozoa undergoing the ionophore-induced acrosome reaction following treatment, however, was higher than that of the controls. The results of sperm-zona pellucida binding studies were equivocal. The findings indicated that the treatment procedure could not be justified for use in IVF, but may be beneficial for intrauterine insemination.