Reports proposing that the cell or origin of "hairy cell" leukemia (leukemic reticuloendotheliosis) is a B lymphocyte have been based primarily on the presence of surface immunoglobulin markers, frequently in "cap" form. Most of the immunoglobulin markers in this series of patients with hairy cell leukemia were multiclass, but were present in cap form under conditions not usually inducing cap formation in normal or leukemic human lymphocytes. In five cases the authors were able to remove these surface immunoglobulins by trypsinization or overnight incubation in serum-free tissue-culture medium. There was no evidence of synthesis of surface immunoglobulins in these cases following incubation in serum-free tissue-culture medium; however, surface immunoglobulins could again be detected after subsequent reintroduction of these hairy cells into the patients' own sera. The peculiar cap formation could not be prevented with sodium azide, a standard inhibitor of cap formation. Phagocytized latex particles and neutral red dye produced similar caplike structures. These findings suggest that hairy cells readily adsorb and pinocytose circulating immunoglobulins, but do not synthesize them.