The chromosome constitution of human spermatozoa was determined after injecting individual spermatozoa into mouse oocytes. Of a total 279 eggs arrested at first cleavage metaphase, 200 (71.7%) were suitable for the analysis of sperm chromosomes. Incidences of spermatozoa with numerical and structural chromosome aberrations were 1.3 and 6.9% respectively in spermatozoa with normal head morphology, showing values comparable with those found in previous studies using the hamster oocyte-human sperm fusion system. The ratio of X- to Y-bearing spermatozoa did not differ significantly from the expected 1:1 ratio. The incidence of structural chromosome aberrations was about four times higher in spermatozoa with amorphous, round and elongated heads (26.1%) than in those with morphologically normal heads, whereas the incidence of aneuploidy was not significantly different between the two groups. No increase in chromosome aberrations was found in spermatozoa with large heads. The same was true for spermatozoa with small heads. Although the sample size used in this study is rather small, the results nevertheless indicate that some morphological abnormalities in the sperm heads are associated with their chromosome defects.