The nature of genetic determinants associated with the regulation of malignancy was investigated in interspecies (human X mouse) cell hybrids. An apparently transformed diploid human cell strain (ME) was fused with a malignant, HGPRT-deficient, mouse cell line (A9). A primary hybrid with an unusual stellate appearance that contained the isozymic forms of lactate dehydrogenase from both parent cells was selected. Through further cloning, four tertiary hybrid cell clones were obtained in which malignancy was substantially inhibited and in which the human chromosome content was reduced or completely absent. Two of the hybrid clones whose malignant phenotype was stably suppressed contained no apparent human chromosomes, whereas the other two, whose phenotype was unstable, contained a part of a human chromosome. Our results are consistent with the view that suppression of malignancy occurs in interspecies hybrids consisting of a malignant mouse and a transformed human parent cell. At present, however, any assignment of a specific human chromosome, or specific fragments thereof, that may suppress the expression of malignancy in these hybrids, is a matter of conjecture.