The effect of Chlorpromazine (CPZ), a membrane active drug, on the morphology and function of blood monocytes and lymphocytes culturedin vitro,was studied. The experiments indicate that CPZ exhibits a strong and dose dependent effect on cell membrane functions like cell adhesiveness and phagocytosis of radiolabelled Candida particles. A biphasic effect was found on the attachment of cells at culture start and on the attachment/engulfment of particles in the cultured cells. Low drug concentrations were found to increase adhesiveness while higher concentrations caused cell lysis. Chlorpromazine in lytic concentrations decreased the intracellular digestion of Candida particles. Small drug concentrations did not appear to influence the digestion of particles as observed morphologically, but a decreased release of metabolized125Todine to the medium was found, suggesting a stabilization of the cell membrane, with reduced transfer of the digested material to the medium.