A diverse group of gonadal and extragonadal human germ cell tumors (GCT) and GCT-derived cell lines was examined for the presence of an i(12p) marker chromosome and/or other abnormalities involving chromosome 12, especially 12p, by bicolor double fluorescence in situ hybridization (FISH). For this purpose three probes, pBS-12, M28, and p alpha 12H8, were used, allowing specific identification of the entire chromosome 12, its short arm, and its pericentromeric region, respectively. The presence of one or more copies of a genuine i(12p) chromosome could be demonstrated in three GCT of the testis, in one ovarian GCT, in one dysgenetic GCT, and in one extragonadal intracranial GCT. Moreover, additional aberrations involving chromosome 12 were shown to be present not only in i(12p) minus but also in i(12p) positive GCT. These data suggest that the occurrence of such aberrations may be a common, although less clearly perceptible and frequent, phenomenon in human GCT.