We describe the production of the monoclonal antibody B73.1, reacting with a subset of human lymphocytes and, in about one-half of the donors, with neutrophilic polymorphonuclear leukocytes. In the peripheral blood from normal adult donors, 14.6 +/- 8.5% of the lymphocytes react with B73.1 antibody. The B73.1(+) lymphocyte subset does not bear markers of typical T or B cells and corresponds to the lymphocyte subset containing antibody-dependent killer (K) and natural killer (NK) cells. We demonstrate that: a) virtually all lymphocytes with K/NK cytotoxic activity are found in the lymphocyte subpopulation bearing the B73.1-defined antigen; b) the B73.1(+) lymphocyte subset bears the combination of antigens known to be present on K/NK cells; and c) there is a positive correlation between the level of cytotoxicity and the actual number of B73.1(+) lymphocytes in individual donors. We also report the distribution of B73.1(+) lymphocytes according to donor age and tissue types. The use of the B73.1 antibody in quantitating the actual number of K/NK cells and in performing functional studies on spontaneous cytotoxicity is discussed.