The effects of erythromycin stearate over a concentration range of 10−6–10−1m on neutrophil functions in vitro were investigated. It was found that erythromycin at concentrations of 10−1–10−2m caused detectable stimulation of chemotaxis to autologous endotoxin-activated serum and inhibition of neutrophil post-phagocytic myelperoxidase mediated protein iodination at concentrations of >5×10−5m. No effects on phagocytosis or post-phagocytic hexose monophosphate shunt activity were observed. In-vivo studies before and after the ingestion of therapeutic doses of erythromycin by six individuals showed no change in neutrophil functions and levels of serum immunoglobulins IgG, IgA and IgM, total haemolytic complement and serum levels of the complement components C3 and C4. However ingestion of erythromycin was associated with increased lymphocyte transformation in five individuals.