This study showed that clofazimine was not radiomimetic, it was not a mutagenic compound, it was not an inducer of prophage-lambda, and it did not eliminate plasmids from appropriate host bacteria. The drug caused an effective inhibition of the growth of Mycobacterium aurum, and also inhibited the growth cycle of the mycobacteriophage D29. Cross-resistance between clofazimine, streptomycin and rifampicin could not be demonstrated. Drug-resistance mutants towards clofazimine could not be isolated, and it was found that the existing clofazimine-resistant strains of Mycobacterium tuberculosis were rather susceptible organisms requiring clofazimine in their growth medium to maintain their drug-resistance. Ultrastructural studies showed that clofazimine did not act by cell wall lysis, nor did it act on bacterial ribosomes. Higher concentrations of the drug resulted in bacterial plasmolysis. These findings are discussed in the light of its known properties and proposed mode of action.