Summary— Ultrastructural studies of spermatozoa from men whose spermatozoa repeatedly had shown poor motility demonstrated an increase in morphological abnormalities. This was considered significant in the 8 men in whom all of the spermatozoa were abnormal and probably significant in those where more than 90% were abnormal (9 men). Spermatozoa from 1 5 fertile men adhered to and penetrated 60% of hamster oocytes, whereas those from infertile men, with both normal and poor semen quality, failed to adhere or penetrate. The heterologous sperm penetration test offers a new approach to the assessment of semen quality in infertile men.