Functional ultrastructural changes in human sperm heads were evaluated following semen dilution in cryoprotective medium (with and without seminal plasma) as well as freezing/thawing. Plasma membranes were as much altered by dilution as by freezing/thawing when compared with fresh samples. The most striking effect on acrosome noted during freezing/thawing was a dramatic decline in percentage of intact spermatozoa. Acrosomal changes seemed to be less important in the fractions without seminal plasma. These deleterious effects were very demonstrable using transmission electron microscopy (TEM), rather than conventional staining. TEM is useful in obtaining more detailed information on membrane and acrosome integrity until a specific procedure to evaluate functional/physical integrity can be found. Cryopreservation in the absence of seminal plasma can be used for intrauterine insemination in AIH, AID programs with frozen semen.