4CMB, 4HMB and BC were assayed for mutagenic activity using the 'microtitre' bacterial fluctuation test without metabolic activation. 4CMB was positive in strains of Salmonella typhimurium detecting both base-substitution and frameshift mutation. BC was weakly positive only in the strain which detects base-substitution mutation. 4HMB was negative in both strains. 4CMB and 4HMB were equally toxic to the strains, whilst BC was comparatively less toxic.