Cytogenetic analysis has been carried out on 17 parallel short-term cultures from four malignant testicular teratomas of intermediate differentiation (MTI), two of them combined with seminomas. Clonal development was seen in three tumors, with most of the variation involving different rearrangements of chromosome 1. Two copies of the i(12p), characteristic of testicular germ-cell tumors, were present in the two tumors with yolk sac elements, a single i(12p) and a 12q- were found in an MTI that had metastasized, and a rearrangement of chromosome 12 containing centromeric chromatin from chromosome 18 was found instead of the i(12p) in a mixed tumor. A 13p+ marker containing Q-negative material was seen in two of the tumors, with a der(7)t(Y;7) in one. Chromosomes 1, 3, 6, 7, 8, 12, 17, and X were invariably overrepresented either as complete or partial trisomies, and chromosomes 4, 5, 13, and 18 were underrepresented in all four tumors, strengthening the idea that tumor suppressor genes on the latter four chromosomes, all of which show some loss of heterozygosity with DNA markers, may be important in tumor progression.