In the present paper the biological activity of methylmethacrylate Monomer and di-methylparatoluidin on stationary, cell-proliferating and bacterial cultures is examined. The equi-toxic concentrations were lower in cell culture systems than in stationary cell suspensions or bacterial cultures. Accordingly biological reaction appears to depend on the effective concentration of the toxic substance as well as on the specific sensitivity of the test system. There is no correlation between the sensitivity of cellular and of bacterial test systems.