Males of the mouse strain B10.BR/SgSn and its congenic mutant strain B10.BR-Ydel, with a partial deletion of the Y chromosome, were used to examine factors related to poor sperm quality and quantity in the mutant strain. The testes of males from the two strains did not differ in their immunohistochemical reaction to androgen receptors or in the number of Sertoli and germ cells in tubules with normal morphology. However, mutants showed a greater frequency of degenerated tubules, a higher level of X-Y chromosome dissociation at meiosis (18% v. 10% in control males), and a lower content of resistant sperm heads in testis homogenates. In the cauda epididymidis, there was a higher percentage of spermatozoa with abnormal heads (88% v. 31%) and of spermatozoa with a cytoplasmic droplet still attached (74% v. 51%). Many sperm heads with flat acrosomes, occurring only in mutants (30% of sperm population), were deficient in proteolytic enzymes, as evidenced by the reaction on gelatine membranes. Most copulations of mutant males (11/18) were sterile in spite of the presence of spermatozoa in the uterus, but in the remaining copulations the fertilization rate was reasonably good (79%). Low numbers of spermatozoa were recovered from the oviducts, and those with the most severely deformed heads were less frequent there than in the uterus. The results show that a partial deletion of the Y chromosome affects efficiency of spermatogenesis, morphology of spermatozoa, their epididymal maturation and capacity to reach the ampulla and fertilize eggs.