Mature sperm and late spermatid are known to be sensitive stages to clastogens in mammalian spermatogenesis. Certain types of chromosomal damage induced in these stages will pass to successive generations as heritable translocations. In the present study, we employed whole chromosome 2 painting with the fluorescence in situ hybridization (FISH) technique to detect the chemically induced translocations in human sperm. Mature human sperm were treated in vitro with an antitumor drug, neocarzinostatin (NCS), and fertilized in vitro with golden hamster oocytes. Sperm pronuclear chromosome slides were prepared at the first cleavage metaphase. To compare the characteristics of translocations between somatic and germ cells, human lymphocytes in peripheral blood treated with NCS in vitro were analyzed at first round metaphase after PHA-stimulation. From the analysis of translocations by whole chromosome 2 painting, frequencies of the haploid genomic translocations (FhG) were predicted for both sperm and lymphocytes. At 1.0 micrograms/ml, the actual percentages of sperm and lymphocytes with chromosome 2 translocations were almost identical (11.9% and 12.0%). At the same dose, however, the FhG of the sperm (1.15) was considerably higher than that of the lymphocytes (0.58), indicating that complex translocations having two or more rearranged sites were induced by NCS more frequently in sperm than in lymphocytes.