A case of hairy cell leukemia in a 39-year-old man is reported. Hairy cells from the peripheral blood, spleen, and bone marrow had lambda-type immunoglobulin on their surfaces; those from the peripheral blood and bone marrow also had IgD on their cell membranes. Frozen sections of spleen reacted with IgGEA, but not IgMEA or IgMEAC markers. Transmission electron microscopy revealed ribosome-lamella complexes in cells from the spleen, but not the peripheral blood. Scanning electron microscopy demonstrated a spectrum of cell surface morphology with many cells characterized by ridges and ruffles. The significance of these findings is considered and it is suggested that the hairy cell is a B lymphocyte.