In fibroblasts and hepatoma cells, interleukin-1 (IL-1) treatment results in the rapid nuclear accumulation of the transcription factor NF-kappaB, present largely as p65 (RelA)/p50 heterodimers. It is well established that this process is dependent in large part upon the phosphorylation and subsequent degradation of the cytosolic inhibitor IkappaB. We looked for other IL-1-induced modifications of NF-kappaB components and found that, in both cell types, IL-1 stimulation led, within minutes, to phosphorylation of both NF-kappaB p65 and p50. Phosphorylation of p65 was sustained for at least 30 min after addition of the cytokine and occurred principally upon serine residues. Immunoprecipitates of NF-kappaB complexes contained an associated protein kinase, the biochemical characteristics of which were indistinguishable from casein kinase II (CKII). Purified CKII efficiently phosphorylated p65 in vitro, apparently on the same major sites that became phosphorylated in intact IL-1-treated cells. Although IL-1 treatment caused little apparent stimulation of total cellular CKII activity, the fraction that was specifically associated with NF-kappaB complexes was markedly elevated by the cytokine. The association of CKII with NF-kappaB occurred in the cytoplasm, suggesting that this phosphorylation might be involved either in control of translocation of the activated complex or in modulation of its DNA binding properties.