Three groups of eggs, intact, cumulus-free and zona-free, were inseminated in vitro with spermatozoa from C57BL/Kw male mice and incubated for 1-2 h. In fixed, toluidine blue-stained preparations, spermatozoa surrounding the eggs or attached to them remained colourless, while sperm heads that had penetrated the vitellus were deeply stained. Those in the early stage with still condensed chromatin were used to evaluate the head shapes. In zona-free and in cumulus-free eggs, only severely misshapen sperm heads appeared at a lower frequency than in the samples used for insemination. The proportion of abnormal sperm heads of all classes was significantly lower in intact than in cumulus-free eggs and most severely abnormal forms were not found in intact eggs. The results suggest that abnormal spermatozoa are able to participate in fertilization, although their chances of penetrating the ova in vitro are greatly reduced in the presence of egg investments, especially of the cumulus.