SummaryUsing Ficoll-Hypaque-separated cells, monocyte chemotaxis was measured by an agarose technique in patients with increased susceptibility to infection, with atopic dermatitis, and in individuals taking aspirin. I n vitro effects of aspirin, hydrocortisone, aminophylline, ephedrine, and diphenhydramine were also studied. Significantly decreased chemotaxis was found in one 9-year-boy with severe mucocutaneous candidiasis and three of 22 patients with atopic dermatitis. In the atopic group of patients greater than 10 years of age, mean monocyte chemotaxis was significantly decreased from the age-matched control group. This decrease did not correlate with serum IgE levels, absolute blood eosinophil counts, or clinical symptom scores. Following aspirin ingestion, mean monocyte chemotaxis significantly decreased whereas neutrophil chemotaxis was unaffected. Using therapeutic concentrations, drug levels of aspirin and aminophylline in vitro caused greater than 35% inhibition of monocyte movement.