Human leukocyte I and i antigens were quantitated using 125I labelled purified antibodies. Binding of these antibodies to leukocytes was dependent on reduced temperature. No significant difference in antigen content was observed between normal and leukemic myeloid leukocytes. B lymphocytes bound much greater amounts of both I and i antibodies than did T lymphocytes. Neoplastic lymphoid cells bound widely divergent amounts of both antibodies with chronic lymphocytic leukemia and lymphosarcoma cell leukemia cells binding much decreased amounts compared to normal lymphocytes. Cells from patients with hairy cell leukemia bound very large quantities of these antibodies in a cold dependent fashion. These elevated levels of binding were not due to nonspecific binding of IgM.