The in vitro activities of GAR-936, the 9-t-butylglycylamido derivative of minocycline, were compared with those of doxycycline, minocycline, acid tetracycline against 527 gram-positive clinical isolates. GAR-936 inhibited all strains, including those resistant to other tetracyclines, at concentrations of less than or equal to 2 mu g/ml, except two strains of JE; diphtheroids for which the MIC was 4 mu g/ml.