Kaposi's sarcoma (KS)-associated herpesvirus open-reading frame (ORF) 73 encodes a latency-associated nuclear antigen (LANA) that is the basis for several serologic assays. Immunoreactive epitopes were searched for by peptide mapping, and 171 cleavable, biotinylated 17-mer peptides offset by 5 residues were synthesized and screened with human serum samples by ELISA. The initial screen, which used highly reactive serum diluted 1:500, identified 38 immunoreactive peptides. These were subsequently tested on additional serum samples diluted 1:40. Thirteen peptides were more reactive with serum samples from patients with KS than with control serum samples. No single epitope was recognized by most KS patient serum samples. Combined use of these peptides did not increase test sensitivity to that of current indirect immunofluorescence assays for LANA (80%-90%). For comparison, full-length ORF73 was expressed in bacteria and analyzed by Western blot. The overall sensitivity was 67% (range, 100% among US patients with classic KS to 52% among Italian patients with classic KS). These studies suggest that LANA immunoreactivity may be due to variations in patient response or conformational epitopes.