The mutagenicity of eleven methylbenz[c]acridines (methyl-B[c]ACRs) was examined by the direct Ames' Salmonella mutagenicity assay on the standard tester strains of TA97a, TA98, TA100 and TA102, each of which is specific to a certain type of mutation. The Benz[c]acridines (B[c]ACRs) studied in this investigation were substituted with one to three methyl group(s) at the 5, 7, 8, 9, 10 and 11 position(s) of B[c]ACR. A certain degree of mutagenic activity was observed by all methyl-B[c]ACRs on at least one of the tester strains, indicating that even non-carcinogenic methyl-B[c]ACRs showed mutagenicity in the assay. Meanwhile, 7,10-dimethyl-B[c]ACR and 7,9,10-trimethyl-B[c]ACR significantly increased the number of back-mutant colonies. However, monomethyl-B[c]ACRs, 7-methyl-B[c]ACR and 10-methyl-B[c]ACR did not increase back-mutation except for TA102 and TA100, respectively. The induction of 7-methyl substituent on B[c]ACR appears to play a key role in frameshift (+1 and -1), base-pair substitution, and single base substitution mutation of the strains in the presence of additional methyl substituent(s) at C-9 and/or C-10 position(s) of D ring of B[c]ACR.