Continuous turbidimetric monitoring of cultures of staphylococci and streptococci exposed to teicoplanin or vancomycin revealed considerable inhibitory activity at concentrations below the conventionally-determined minimum inhibitory concentration. Teicoplanin was more active than vancomycin against low inocula, but exhibited a larger inoculum effect. A modest decline in susceptibility to teicoplanin and vancomycin could be induced by sequential exposure to the drugs. Such variants gradually reverted to susceptibility on passage in antibiotic-free broth. The morphological consequences of exposure to the two antibiotics were similar as judged by scanning electron microscopy.