In the present study, we evaluated whether mononuclear leukocytes could synthesize and secrete growth hormone (GH) in vitro. By using RNA slot blot analysis, we detected maximum spontaneous levels of specific GH mRNA in the cytoplasm of rat leukocytes after a 4-h incubation. Northern gel analysis demonstrated that the specific leukocyte GH RNA was polyadenylated and had a molecular mass of 1.0 kb. Further studies using immunofluorescence, antibody affinity chromatography, and Sephacryl gel filtration indicate that leukocytes secrete a high molecular weight (greater than 300,000) and a low molecular weight (approximately 22,000) immunoreactive GH (irGH). A substantial amount of the high molecular weight irGH can be converted to the lower molecular weight form after reduction with mercaptoethanol. The irGH appeared to be de novo synthesized because it could be radiolabeled with tritiated amino acids and its production could be blocked by previous incubation of leukocytes with cycloheximide. The replication of Nb2 rat node lymphoma cells was stimulated by affinity-purified human lymphocyte-derived irGH. The growth stimulation was blocked by specific antibodies to hGH. We conclude that lymphocytes produce an irGH that is similar to if not identical to pituitary GH in terms of bioactivity, antigenicity, and molecular weight. The findings demonstrate a potential regulatory loop between the immune and neuroendocrine tissues.