Human peripheral blood lymphocytes (PBL) contain immunoreactive growth hormone-releasing hormone (irGRH). Both the intracellular content and the release of irGRH from PBL are increased following lymphocyte activation. Although binding of lymphocyte-derived irGRH to a GRH1-44 antibody dilutes in parallel with standard GRH1-44, irGRH elutes earlier on gel chromatography with an apparent molecular weight of approximately 50 kD. On reverse-phase high performance liquid chromatography, irGRH elutes later than GRH1-44. Northern blot analysis also detected the presence of two GRH mRNA transcripts in PBL, one similar in size to the hypothalamic species of 0.75 kb and another larger transcript of approximately 10 kb. Despite these differences in physicochemical behaviour, both the lymphocyte-derived irGRH GRH1-44 were biologically active as demonstrated by enhancing the transcription of growth hormone mRNA both in dispersed rat pituitary cells and in PBL.