The effect of phenylbutazone and chloramphenicol on the function of blood monocytes culturedin vitrowas studied. Both drugs had an inhibitory effect on the engulfment stage of phagocytosis. While a moderate effect only of chloramphenicol on digestion of engulfed yeast particles was found, phenylbutazone caused a marked reduction of the digestion of engulfed particles. Concomitant with this reduction of digestive ability, the lysosomes showed no morphological alterations as observed in control cultures without drug addition, indicating a lack of fusion between the engulfed particles and lysosomes in the presence of phenylbutazone.