Previous studies have shown that erythromycin can enter phagocytic cells, stimulating their functional activity. In this work we compared the effects of erythromycin and another newer macrolide antibiotic, miocamycin, on a series of in vitro tests aimed at evaluating their influence on polymorphonuclear cell (PMN) functions. Results indicate that erythromycin induces an increase in leukotriene B4 production in PMNs, while chemotaxis, killing of Candida albicans and respiratory burst are not influenced, at least at the doses used in this study. On the contrary, all these activities are significantly enhanced following incubation with miocamycin, and the response varies according to the antibiotic concentration.