The mutagenic activities of five 1-alkyl derivatives of 3-nitro-1-nitrosoguanidine, viz. methyl, ethyl, propyl, isopentyl and octyl, were tested on Streptomyces indicus. All the compounds were found to be mutagenic. The methyl and ethyl derivatives were most effective regarding mutational frequency. The effectiveness decreased with increasing number of C-atoms in the alkyl group of the mutagens. The compounds tested had a much higher mutagenic efficiency that UV-rays. A marked variation existed in antibiotic activity among the biochemical mutants: increase, decrease or complete absence of such activity were seen.