The levels of c-myc protein expression in three types of Epstein-Barr virus (EBV) transformed human B-cell derived lines were examined with an ELISA assay. Six independently maintained sublines of the same EBV-transformed pro-B-cell line (FLEB-14), six B-cell lines (LCL) and six Burkitt's lymphoma lines (BL) were compared. The average amount of c-myc protein, calculated from at least three independent tests on each line, differed between the three groups. Expressed in relative units, the ratio of the means was 1:2:5 for the LCL:FLEB:BL lines. The differences were statistically significant at P less than 0.01.