Intercellular, nonartifactual variability of nucleolar organizer region (NOR)-Ag-staining was studied in cultured human peripheral blood lymphocytes, skin and embryonic fibroblasts. No differences in number and character of variable NORs and intensity of their staining were observed between lymphocytes stimulated to proliferate with phytohemagglutinin and pokeweed mitogen, as well as lymphocytes of first- and second division. The number of NOR associations per cell and the number of associated chromosomes per association were also similar. In a given individual these criteria were similar in lymphocytes and fibroblasts. In all nine clones derived from three independent parental fibroblast cultures the intercellular NOR-Ag-variability was similar to that observed in a given parental cell line. A significant decrease in the number of metaphases containing NOR associations was observed in second-division lymphocytes compared with first-division ones, as well as in skin fibroblasts compared with lymphocytes.

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