4-(9-Acridinylamino)methanesulfon-m-anisidide (m-AMSA), an antitumor agent, is a potent clastogen in a variety of mammalian cell lines, but it is only weakly mutagenic at the hprt locus (Cancer Res. 44:4420, 1984; 45:3143, 1985). To better understand the apparent inability of m-AMSA to induce gene mutations while being clastogenic, the authors investigated the mutagenic and clastogenic abilities of m-AMSA and a congener, o-AMSA, at the tk locus in L5178Y/TK+/- -3.7.2C mouse lymphoma cells. m-AMSA and O-AMSA were highly mutagenic at the tk locus. Between 90-95% of the TK mutants at all doses were small-colony mutants, indicating that most mutations were due to chromosomal events. The potent clastogencity of these agents was confirmed by gross chromosome aberration analysis. The high correlation between the clastogenic potency and mutagenic potency of m-AMSA and o-AMSA at the tk (but not at the hprt) locus may be because these compounds induce chromosomal rearrangements and/or deletions that affect the expression of multiple loci. If any of the affected linked genes are essential genes, the loss of function may be lethal in the hemizygous state; and thus, these mutants may not be recovered at the hprt locus but may be recovered at a heterozygous locus like tk.