The total serum protein synthesis, amino acid assimilation, rate of cell division, and DNA synthesis with various concentrations of methyl methacrylate monomer in medium was determined in a culture of L 929 fibroblasts of the mouse. DNA synthesis was inhibited after 12 hours of incubation. The normal course of cell division was completely supressed with a concentration of 3.2 . 10(-2) mol/l. A decrease in protein synthesis was observed after 24 hours of incubation. This decrease may be considered the result of DNA inhibition.