A radioimmunometric method was developed for the quantification of lactoferrin molecules natively bound to blood monocyte and lymphocyte surfaces and the estimation of the surface lactoferrin-binding capacity of these cells after their incubation with exogenous lactoferrin. Values of surface lactoferrin obtained were greatest for monocyte-rich isolates (9,168 ± 1,713 molecules/cell; n = 19). The values of monocyte surface lactoferrin for males were similar to those of premenopausal females (8,980 ± 2,378 (n = 8) and 9,427 ± 2,606 molecules/cell (n = 11), respectively), but males had slightly lower values of monocyte surface lactoferrin binding capacity than did premenopausal females (10,447 ± 2,478 molecules/cell versus 15,958 ± 3,731 molecules/cell, respectively; p > 0.05). Expressed as saturation of the monocyte surface lactoferrin binding capacity, values of 97.2% ± 22.6% for males and 76.6% ± 14.3% for females were calculated. Intermediate values of surface lactoferrin were found in B-lymphocyte-rich isolates from five patients with B-cell chronic lymphocytic leukemia. In T-lymphocyte-rich preparations, there were low levels of native lactoferrin expression (154 ± 63 molecules of lactoferrin/cell; 3 isolates). The present technique should permit additional quantitative studies of mononuclear cell surface lactoferrin to determine the role of lactoferrin surface binding and analyses of factors that modulate this binding.