To investigate the expression of human herpesvirus 8 (HHV8)-encoded proteins in the cells of primary effusion lymphoma (PEL), Kaposi's sarcoma (KS) and multicentric Castleman's disease (MCD), nine rabbit polyclonal antibodies to K2, ORF26, K8, K8.1, K10, K11, ORF59, ORF65, and ORF73 were developed. Western blot analysis in PEL cell lines (TY-1 and BCBL-1) revealed that the expression of these proteins, except ORF73 (LANA), was induced by tetradecanoylphorbol acetate (TPA) treatment, indicating that these proteins are lytic proteins. Immunofluorescence assay in primary PEL cells derived from pericardial effusion and PEL cell lines with and without TPA treatment revealed that primary PEL cells exhibited the same expression pattern as noninduced PEL cell lines, and the treatment changed localization of K8, ORF59, and ORF65 proteins. Immunohistochemistry revealed that 90% of KS spindle cells expressed the ORF73 protein, whereas a small population of KS cells expressed K8, K10, K11, ORF59, and ORF65 proteins. In MCD, ORF73, ORF59, K8, K2, and K10 proteins were expressed in the cells at mantle zone of the follicle. These data indicate that KS and PEL cells expressed predominantly latent proteins, whereas MCD expressed both latent and lytic proteins, suggesting that HHV8 plays a different role in the pathogenesis of HHV8-associated diseases.