The effect of zinc and erythromycin on cultures inoculated with mixtures of different ratios of erythromycin sensitive (ES) and resistant (ER) Propionibacterium acnes cells was studied in vitro. Propionibacterium acnes ES outgrew P. acnes ER in the absence of erythromycin and zinc. At low levels of erythromycin ES outgrew ER, whilst the addition of 600 pg/ml zinc further reduced the growth of ER compared to ES. Growth of ER and ES were similar at levels of erythromycin near the minimum inhibitory concentration (MIC) of ES cells. Concentrations above the MIC for ES cells inhibited ES but not ER cells. At the higher concentrations of erythromycin, the addition of 96 ng/ml zinc delayed the growth of ER cells, whilst the addition of 300 micrograms/ml zinc prevented the growth of ER cells. The combination of erythromycin and zinc, at appropriate concentrations, inhibits both ES and ER.