Hairy cells from eight patients with hairy cell leukemia were evaluated with both light and transmission electron microscopy for their capacity to phagocytose zymosan, latex, staphylococcus aureus, and pseudomonas aeruginosa. In two patients, there was no phagocytosis of any of these substances; cells from three patients phagocytosed only latex; two, all except pseudomonas; and one, all 4 substances. Hairy cells became relatively smooth while in culture with staphylococcus, but no surface changes were noted during incubation with the other substances. Of the eight patients studied, one died of pseudomonas pneumonia and sepsis; pseudomonas was the only substance which her hairy cells did not phagocytose. The one patient whose hairy cells phagocytosed all 4 test substances developed a disseminated Mycobacterium intracellulare infection; culture of his hairy cells with this atypical myocbacterium showed no phagocytosis. Hairy cells have different phagocytic capabilities from patient to patient, and the evaluation of these capabilities in vitro might provide early identification of potential infectious complications.