The aim of the present work was to examine the sensitivity of human spermatozoa from oligospermic and normospermic semen samples to manipulations in the course of isolation by generally used procedures. To this end we performed protein and DNA estimations in spermatozoa isolated by procedures differing in rigorousness (washings with Ringer's buffer and a medium containing ficoll) as well as microscopic examinations of washed sediments. The results showed that the washed sediments contained, in addition to spermatozoa, materials containing protein and DNA, possibly deriving from the breakage of sperm cells. These materials increased in amount with the decrease of original sperm counts. We suggest that abnormal spermatozoa, whose percentage usually increases with the severity of oligospermia, are more sensitive to damage than the normal ones.