A quantitative procedure to characterize the affinity of human natural killer (NK) cells for K562 tumor is described. Using highly purified (greater than or equal to 98%) NK cells, measurements of the conjugate frequencies permit the determination of the apparent Michaelis constants (KappM) for the conjugation process. Because no intermediate steps for the lytic process are involved the interpretation of the values of KappM is the simplest one that can be achieved. Thus, we found that a plot of KappM against the number of effector cells allows us to determine the dissociation constant, KS, that characterizes the effector-target affinity. KS is independent of the donor cell source and this value was (1.0 +/- 0.1) x 10(5) cell/tube. In contrast, the KappM values vary among donors, and this could be used to compare the relative activity of different donors in relation to their binding capacity.