When rat fibroblasts (Ad12-3Y1-Z19) transformed with adenovirus type 12 were cultured with lignin derivative (acetyl or sulfonyl), the cells grew for 2 to 3 days at the same rate as the control cells cultured without lignin derivative, then rapidly died. This cytocidal effect was independent of the cell population density. The lag time was longer than the doubling time (-24 h) of Ad12-3Y1-Z19 cells. Other polyanions such as dextran sulfate and glycosaminoglycans did not show significant inhibitory effect on Ad12-3Y1-Z19 cell growth. In order to determine whether or not this cytocidal effect is general for every cell line, we examined 14 cell lines derived from tumor tissues and normal tissues, and 8 cell lines transformed with viruses, chemical carcinogens, or oncogenes. Of these cell lines, many responded to lignin derivatives with inhibition of cell growth, while in some cell lines no inhibitory effect of lignin derivatives was observed. The cytocidal effect was observed in only Ad12-3Y1-Z19 cells. This may be a new type of cytocidal phenomenon.