The technique of in situ hybridization was used to examine human GH gene expression in human GH-secreting pituitary adenoma cells. Five somatotroph adenoma specimens obtained at surgery were dispersed into single cell suspensions. Hybridization experiments were performed on cells immediately after dispersal or on cells cultured for 48-72 h in a defined serum-free medium. Tritiated GH cDNA was used to probe fixed cells, and hybridization was determined by liquid autoradiography. Of the freshly dispersed adenoma cells probed with GH cDNA, more than 70% contained GH mRNA, as determined by counting silver grains per cell. Significant cellular grain counts were obtained for GH cDNA-probed cells from all five tumors, while negative controls showed negligible silver grain counts. In cultured cells derived from three of five tumors, an average of 40% contained detectable GH mRNA. Therefore, quantitative in situ hybridization is a useful technique to demonstrate the expression of GH mRNA in human pituitary adenoma cells.