Growth hormone (GH) exerts its regulatory functions in controlling metabolism, balanced growth and differentiated cell expression by acting on specific receptors which trigger a phosphorylation cascade, resulting in the modulation of numerous signalling pathways dictating gene expression. A panel of five monoclonal antibodies was used in mapping the presence and somatic distribution of the GH receptor by immunohistochemistry in normal and neoplastic tissues and cultured cells of human, rat and rabbit origin. A wide distribution of the receptor was observed in many cell types. Not all cells expressing cytoplasmic GH receptors displayed nuclear immunoreactivity. In general, the relative proportion of positive cells and intensity of staining was higher in neoplastic cells than in normal tissue cells. Immunoreactivity showed subcellular localisation of the GH receptor in cell membranes and was predominantly cytoplasmic, but strong nuclear immunoreaction was also apparent in many instances. Intense immunoreactivity was also observed in the cellular Golgi area of established cell lines and cultured tissue-derived cells in exponential growth phase, indicating cells are capable of GH receptor synthesis. The presence of intracellular GH receptor, previously documented in normal tissues of mostly animal origin, is the result of endoplasmic reticulum and Golgi localisation. Heterogeneity of immunoreactivity was found in normal and neoplastic tissue with a variable range of positive cells. The nuclear localisation of immunoreactivity is the result of nuclear GH receptor/binding protein, identically to the cytosolic and plasma GH-binding protein, using a panel of five monoclonal antibodies against the GH receptor extracellular region. The expression of GH receptors, not only on small proliferating tumour cells such as lymphocytes, but also on well differentiated cells including keratinocytes, suggests that GH is necessary not only for differentiation of progenitor cells, but also for their subsequent clonal expansion, differentiation and maintenance.