The specific silver staining of nucleolar region organizers was applied to Daudi lymphoma control and interferon alpha-treated cells. Isolated nuclei from control and treated samples were used for the kinetic analysis of in vitro RNA and DNA synthesis. Results have shown that interferon treatment induces a reduction of the transcriptional and replicative activities within 90 min. Concomitant to these results is the modification of the organization of nucleoli. Intensity and distribution of silver grains are, in fact, different in treated cells nucleoli as compared to those of controls. Thus, the number and the arrangement of granules could be related to the functional state of the cells suggesting that the transient cascade of interferon-generated signals involves also modulation of nucleolar structure and function in accordance with the hypothesis of a relationship of cell proliferation rate to silver-stained nucleolar protein quantity.