10 of 11 clinical isolates of Serratia marcescens yielded small numbers (10(-7) to 10(-8)) of 'gray' colony-type variants after selection with either amikacin, gentamicin or kanamycin, of which most proved resistant against all of the following aminoglycoside antibiotics: amikacin, gentamicin, kanamycin, neomycin, netilmicin, sisomicin and streptomycin. The level of resistance was not absolute, but rather low-level to intermediate. All except two of these 'gray', resistant phenotypic variants yielded 'opaque' revertants which were essentially indistinguishable from the parent (wild type) strains in terms of colonial morphology and antibiotic susceptibility. The 'gray' variants kinetically were more susceptible to the bactericidal activity of fresh human serum than the 'opaque' variants. Preliminary evidence afforded the tentative conclusion that this nonspecific aminoglycoside-resistance mechanism was not plasmid-mediated.