Knowledge on the antibiotic suceptibility of pathogenic fermentative actinomycetes is still fragmentary and contradictory, which is one reason why even today the chemotherapy of human actinomycosis may still present problems. Complete and reliable data on the in vitro sensitivity of these pathogens can be obtained, however, when standardized and specially adapted methods of susceptibility testing are employed. Special adaptation should cover the general technique of in vitro testing, as well as the choice of test media, the preparation of inocula, incubation methods and method of reading results. Provided that all factors are taken into account which might cause difficulties in test reproducibility and therapeutic relevance of the results, in vitro tests appear to contribute to the revision and updating of treatment regimes for actinomycosis. Our findings indicate that, apart from ampicillin the clinical efficacy of which has already often been proved, mezlocillin, cefoxitin, clindamycin, tetracyclines, rifampicin and erythromycin may be recommended as alternative drugs which would seem to offer good chances of success.