Differential activity of rRNA gene clusters following growth-hormone administration has been demonstrated in cultured lymphocytes from subjects with different genetic backgrounds, i.e., with or without in vivo peripheral responsiveness to the hormone. The influence of different culture conditions on ribosomal gene responsiveness was also tested. Ribosomal geneactivity was evaluated by selective silver staining of nucleolus organizing regions. The results show that hormone-induced enhancement of transcriptional activity requires both genetically determined cell responsiveness and environmentally determined permissive factors.