Human sperm chromosomes from a 46,XY/47,XXY male were obtained using the technique of in vitro penetration of zona-free hamster eggs. The analysis of 543 sperm complements shows a significantly increased incidence (0.9%) of hyperhaploid gonosomal 24,XY sets, with a lack of the expected corresponding gonosomal hypohaploidies, and a normal rate of autosomal non-disjunctions. These results support the suggestion that 47,XXY cells are able to go through meiosis and to form spermatozoa. Only 24,XY sperm chromosomal constitutions were observed suggesting a preferential pairing of homologous sex chromosomes in 47,XXY spermatocytes.