Cytogenetically, a marker chromosome interpreted as i(12p) is present in most testicular tumors of germ cell origin. In this study, 22 patients with testicular germ-cell tumors were investigated by Southern blot hybridization to characterize changes in chromosome 12. In comparison with normal DNA, tumor DNA of 18 patients showed increased dosages of 12p accompanied by a comparable or smaller increase or no change in the dosage of centromeric sequences of chromosome 12. A likely interpretation was that most testicular tumors had one or several isochromosomes for 12p that were formed by somatic division of the centromere and that the points of breakage and reunion in the centromeric region were different in different tumors. Allelic 12p fragments showing increased intensity were paternal in four and maternal in three of seven informative cases. Thus, there was no evidence of sex-limited parental imprinting. Furthermore, the observed patterns of allelic fragments suggested that the marker was an i(12p) formed by sister chromatids of one homolog number 12 rather than the result of interchange of genetic material between different homologues.