The phosphorylation of the proteins of heterogeneous nuclear ribonucleoprotein particles has been investigated in HeLa cells. 32Pi labeling of intact cells indicated that, of the six major particle proteins, the most heavily phosphorylated was the C1-protein (Mr = 42,000). This protein, together with C2 (Mr = 44,000), is also phosphorylated by [gamma-32P]ATP in particle extracts and in particles that are purified by sedimentation or exclusion chromatography. The C-proteins, together with their particle-associated kinase, were partially purified from isolated particles following dissociation with micrococcal nuclease. Proteins C1 and C2 co-purify on phosphocellulose chromatography, and their peak overlaps with that of a casein kinase activity. Evidence suggesting that this kinase activity is responsible for C-protein phosphorylation includes 1) the phosphorylation of C-proteins in the fractions where they overlap with the kinase, 2) the phosphorylation of added C-protein by fractions of the casein kinase which lack detectable C-protein, and 3) the similarities in catalytic properties of the casein kinase- and C-protein-phosphorylating activities. The purified kinase activity is cyclic nucleotide and Ca2+ independent. It is stimulated by polyamines, inhibited by heparin, and utilizes either GTP or ATP with high affinity. Serine residues are the major phosphate acceptors. These properties indicate that the kinase is casein kinase II or a closely related enzyme. Moreover, purified casein kinase II from rabbit liver effectively phosphorylates C-protein. These results suggest that C-proteins may be natural substrates for nuclear casein kinase II.