The minimum inhibitory concentrations for erythromycin, clindamycin, lincomycin, tetracycline and minocycline have been determined for 92 clinical and three culture collection isolates of Actinomyces. From a consideration of MIC values and expected serum levels from oral therapy, minocycline was the drug of choice for the treatment of actinomycosis in patients allergic to penicillin. The serum levels of six patients allergic to penicillin, treated with oral minocycline 1 g/day were monitored and found to exceed the MIC for the Actinomyces species responsible for the condition. In all six Actinomycosis cases resolution was achieved in 8-16 weeks of oral minocycline therapy with no recrudescence for 1 year.