The protein immunoblot technique was used to identify Epstein-Barr virus-specific antigens present in sodium butyrate-induced P3HR-1 cells. Using sera from patients with either nasopharyngeal carcinoma or arthritis, 16 polypeptides were detected ranging in molecular weight from 22K to 140K. Each of the anti-EA-, anti-VCA-positive sera were found to contain antibodies to different subsets of the antigens. A 72K protein was identified which was consistent with the nuclear antigen (EBNA), and culturing cells in the presence of disodium phosphonoacetate allowed identification of 140K and 22K antigens as late viral products. Treatment of cells with sodium butyrate revealed that expression of some antigens increased in parallel with the time of incubation of the cells in butyrate while other antigens either appeared early and then decreased in intensity or were only present after a number of days of butyrate treatment. One of the antigens which decreased with the time cells were treated with butyrate was EBNA.