Summary: The infection of slow-growing one-step intermediate streptomycin- resistant mutants with phage from wild-type bacteria, indifferent (one-step completely resistant) or other one-step intermediate resistant mutants gives rise to fastgrowing streptomycin-sensitive transductants. From this fact it is clear that both slow growth and streptomycin resistance are controlled by a single locus. The transductants are very easily detected on the poor background growth of the intermediate resistant recipient. With this technique it has become possible to cross one-step resistance with resistance, indifference and sensitivity. The results so far obtained indicate that one-step intermediate resistance is controlled by multiple loci. None of these loci is linked to the indifference locus. The fast-growing mutants which develop from the two-step resistant mutants are assumed from the transduction analysis to be due to reverse mutations at the first step loci.