SummaryMinocycline, a new tetracycline, inhibitsin vitromost strains of pneumococci, streptococci (except enterococci) and staphylococci at concentrations of 0.5 to 1.5μg/ml, which are easily maintained in the blood of patients treated with daily doses of 200 mg of minocycline.The minimum inhibitory concentrations of minocycline for most Enterobacteriacae are higher, but the antibacterial activity of minocycline can be greatly increased by adjustment of the pH at the acid level.Minocycline has been clinically effective in the treatment of infections in 35 (77%) out of 45 patients. No adverse effects were observed. Failures of therapy were caused by coexistence of a neoplastic lesion (5 cases), by superinfection due to minocycline-resistant organisms (4 cases) and by failure of minocycline-resistant pathogens to respond to treatment (1 case).