Studies were performed to prove or disprove the specificity of binding of HGH [human growth hormone] to human lymphocytes by investigating the displacement of 125I-HGH by unlabeled hormone. 125I-HGH was bound to normal human lymphocytes, depending on the incubated number of cells from 0.52 .+-. 0.11 to 1.07 .+-. 0.09%. Binding of 125I-HGH was increased (P < 0.01) on preincubation with PHA [phytohemagglutinin] and ranged from 2.36 .+-. 0.38 to 4.32 .+-. 0.56%. Lymphocytes from patients with CLL [chronic lymphocytic leukemia] bound 0.29 .+-. 0.07 to 0.59 .+-. 0.14% of 125I-HGH, depending on the respective number of the cells incubated. Binding of 125I-HGH to CLL-lymphocytes was significantly (P < 0.05) lower than that of unstimulated lymphocytes from healthy subjects. No displacement of 125I-HGH was observed on administration of an excess of unlabeled HGH in all cell specimens studied.