We designed a short-term in vitro assay for detecting tumor promoters, utilizing the activation of Epstein-Barr virus (EBV) expression in EBV genome-carrying human lymphoblastoid cells. This system is composed of EBV-non-producer Raji cells as the indicator, n-butyrate as the EBV-inducer, and the test substance. After addition of the latter 2 components to the culture medium, the cells are cultivated for 48 h at 37 degrees C and the ratio of EBV early antigen (EA)-expressing cells was assessed using immunofluorescence. This assay system allows for a rapid detection of the activity of the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and its related compounds and also of the Euphorbiaceae plant extracts containing such active principles. Among several microbial products tested, teleocidin, an indole-alkaloid produced by a Streptomyces species, was also detected and had an activity level comparable to that of TPA. Other promoters, such as anthralin, phenol, Tween 60 and 80 and the carcinogenic ("initiator") substances including benzopyrene, did not react with the system. The test is simple to perform, reproducible and should be applicable for mass-screening of promoter substances in the environment.