Established human lymphoid cell lines bound calcitonin with high affinity. Calcitonin binding was detectable on both B cell and T cell lines derived from patients with a spectrum of lymphoproliferative disorders. These cell lines had marked differences in binding capacity per cell even when adjusted for differences in surface area. The binding was linearly related to cell density and showed a complex temperature dependency. Steady state kinetic analysis suggested a single class of high affinity receptors with an apparent association constant of 4 x 10¹⁰m^-1. Competitive binding studies with calcitonin analogs revealed an order of K_i values similar to those previously obtained using plasma membranes from known calcitonin target tissues. Human calcitonin, as found previously with renal or skeletal membranes from the rat, competed only weakly for the lymphocyte receptors confirming that the human analog has evolved into a weak or unstable form, or both.