We have developed a qualitative method to assay for the presence of 2-deoxystreptamine in hydrolysates of crude aminoglycoside preparations using a 2-deoxystreptamine-requiring idiotrophic mutant. The assay involves (i) incubation of a 2-deoxystreptamine-requiring mutant with 1 mg of a hydrolyzed preparation from a crude unknown antibiotic mixture per ml, and (ii) examination of the resultant incubation mixture for production of antibiotic(s) by disk assay.