The nucleolar acid-soluble proteins from normal rat liver and Novikoff hepatoma ascites cells were labeled in vivo for 2 hr after the injection of [32P]orthophosphate and in vitro with [gamma-32P]adenosine triphosphate in systems containing 0.25 M sucrose, 5 mM MgCl2, 12.5 mM NaCl, and 0.05 M Tris-HCl buffer, pH 7.3, AT 37 DEGREES. Two-dimensional polyacrylamide gel electrophoresis and autoradiography showed that approximately 40 and 20 protein spots were labeled in vivo and in vitro, respectively. There were some marked differences in labeling in vivo between normal rat liver and Novikoff hepatoma acid-soluble nucleolar proteins. By 32P analysis of gel slices, the proportion of the total 32P incorporated into protein Spot A1-4 was greater in normal liver, and the proportion of 32P incorporated into some high-molecular-weight protein spots, such C23-24 and C26-27, was greater in the Novikoff hepatoma ascites cells. With the in vitro incubation system, the 32P uptake per mg protein was about twice as high in Novikoff hepatoma nucleolar proteins as in normal rat liver nucleolar proteins but, generally, the same proteins were labeled in both tissues.