We have previously shown that the 12-kDa capsid protein (p12) of herpes simplex virus type 1 (HSV-1) is a gamma 2 (true late) gene product encoded by the UL35 open reading frame (D. S. McNabb and R. J. Courtney, J. Virol. 66:2653-2663, 1992). To extend the characterization of p12, we have investigated the posttranslational modifications and intracellular localization of the 12-kDa polypeptide. These studies have demonstrated that p12 is modified by phosphorylation at serine and threonine residues. In addition, analysis of p12 by acid-urea gel electrophoresis has indicated that the protein can be resolved into three components, designated p12a, p12b, and p12c. Using isotopic-labeling and alkaline phosphatase digestion experiments, we have determined that p12a and p12b are phosphorylated forms of the protein, and p12c is likely to represent the unphosphorylated polypeptide. The kinetics of phosphorylation was examined by pulse-chase radiolabeling, and these studies indicated that p12c can be completely converted into p12a and p12b following a 4-h chase. All three species of p12 were found to be associated with purified HSV-1 virions; however, p12b and p12c represented the most abundant forms of the protein within viral particles. We have also examined the intracellular localization of p12 by cell fractionation and indirect immunofluorescence techniques. These results indicated that p12 is predominantly localized in the nucleus of HSV-1-infected cells and appears to be restricted to specific regions within the nucleus.