Control of cytokine activity is provided in certain cases by the generation of soluble forms of the receptor, which mainly function as binding proteins, although some other agonistic properties have been described. The objective of the present study was to identify the growth hormone binding protein (GHBP) and the prolactin binding protein (PRLBP) in human serum. GHBP was partially purified by a new method using Sepharose-immobilized hGH and separated and detected by electrophoresis and immunotransference (Western blot) using an antibody against the GH receptor. We found that human serum GHBP is a complex mixture of proteins with molecular weights in the range of 27 to 62 kDa, structurally related to the receptor and whose origin is still unknown. Sera from breast cancer patients and pregnant subjects showed the same protein profile although the levels were lower than the normal ones. those in pregnant subjects being the lowest. A 150 kDa protein, which dissociates into two subunits of 50 and 30 kDa, was identified by an anti-PRL receptor antibody, consistent with previous reports. According to the molecular weight and the lack of hGH binding activity observed, it is likely that the protein is nota soluble form of the receptor but rnight be an anti-idiotipic antibody in human serum. No differences in the protein profile were observed in the three sera investigated. The results suggest the presence of a prolactin binding protein, related to the receptor and whose origin awaits further studies.