Sperm chromosome complements were studied in a man heterozygous for a reciprocal translocation t(9;13)(q21.1;q21.2). A total of 89 spermatozoa were karyotyped after in vitro penetration of hamster eggs. The frequencies of alternate, adjacent 1 and adjacent 2 segregations were 46.9%, 35.8% and 17.3% respectively. For alternate segregation, the number of normal spermatozoa (21) was not significantly different from the number of spermatozoa carrying a balanced form of the translocation (17), as theoretically expected. The proportion of spermatozoa with an unbalanced form of the translocation was 53.1%. There was no evidence for an interchromosomal effect since the frequency of numerical abnormalities (unrelated to the translocation) was within the normal range of control donors. Data from a total of 31 reciprocal translocations studied by sperm chromosomal analysis were reviewed.