Ellipticine is a potent clastogen in CHO cells (Bhuyan et al: Cancer Res 32:2538-2544, 1972). The reported mutant frequencies produced by ellipticine at the hprt locus in CHO cells are less than or equal to 50/10(6) survivors (background approximately 2/10(6); survival = 10%) (DeMarini et al: Cancer Res 43:3544-3552, 1983; Singh and Gupta: Cancer Res 43:577-584, 1983; Environ Mutagen 5:871-880, 1983). In the present study, the mutagenic and clastogenic activities of ellipticine were evaluated in L5178Y/TK(+/-)-3.7.2C mouse lymphoma cells. Unlike the results at the hprt locus, ellipticine is a potent mutagen at the tk locus, with as little as 50 ng/ml producing an induced mutant frequency of 142/10(6) survivors (background = 56/10(6); survival = 61%) and 198/10(6) survivors (background = 72/10(6); survival = 50%) in two separate experiments. This same dose of ellipticine induced 44 aberrations per 100 metaphases (background = 5/100 cells). At 400 ng/ml, ellipticine induced over 1,000 mutants/10(6) survivors at approximately 10% survival and produced 242 aberrations/100 cells. Under the test conditions, most of the aberrations were chromosome rather than chromatid events. As expected for a compound acting primarily by a clastogenic mechanism, almost all of the TK-deficient mutants were small colonies. Thus, ellipticine is a potent clastogen in both Chinese hamster cells and in mouse lymphoma cells; however, it is a potent mutagen at only the tk locus and not at the hprt locus. These results support the hypothesis that the location of the target gene affects the ability of the assay to detect both intragenic events and events causing the loss of multiple loci. Thus, a heterozygous locus (like tk) but not a functionally hemizygous locus (like hprt) may permit the more efficient detection of mutagens that act primarily by a clastogenic mechanism.