For purposes of studying the degree of polymerization of actin in nuclei, nuclei from 35S-labeled amoebas (Amoeba proteus) were transplanted into unlabeled cells, which were immediately lysed and extracted under conditions considered to stabilize preexisting fibrous actin. The enucleated 35S-donor cells were similarly treated for analysis of cytoplasmic actin. The extraction conditions permitted separation of soluble (unpolymerized or G) actin from pelletable (polymerized or F) actin, and the radioactivity of each was determined after the actin was separated from other proteins by polyacrylamide gel electrophoresis. We found that about 2/3 of the actin within the nucleus is pelletable, whereas only about 1/3 of the cytoplasmic actin is pelletable. We speculate that polymerized actin in the nucleus is involved in the condensation of chromatin.