We examined eight human germ cell cancer lines (GCCLs) for cytogenetic abnormalities and found an isochromosome 12p, i(12p), marker in all seven male nonseminoma GCCLs, but not in the single female teratocarcinoma cell line. Southern blot analysis of these cell lines showed increased copy number for c-ki-ras2, a gene located on 12p, in all the male GCCLs. The comparison of Southern blot analysis for a restriction fragment length polymorphism (RFLP) probe localized to 12p to a probe for int-1, which maps to 12q, indicates that the increased copy number for c-ki-ras2 is primarily from the greater numbers of 12p relative to 12q. Although Northern analysis revealed enhanced mRNA expression for c-ki-ras2 in the GCCLs with an i(12p), hybridization of specific end-labelled oligonucleotides to the polymerase chain reaction products of c-ki-ras2 codons 12, 13, or 61 did not identify c-ki-ras2 mutations of these codons in these cells. Thus, c-ki-ras2 activation through point mutation is an infrequent event in GCCLs. These data further suggest that increased 12p copy number is a common event in the transformation process leading to male germ cell cancer. We conclude that determination of 12p copy number by cytogenetic analysis or Southern blotting is useful in the diagnostic evaluation of human germ cell cancer.