Toxicity of drugs cultured cell line (RK12-, EL-cell line) were exposed to various concentrations/ml medium for 48 hours and for 14 days. The morphologic changes of both cell lines observed included granulation and shrinkage of the cytoplasm, formation of long and narrow cytoplasmic projection, and appearance of giant like cells. ID50 values of ibuprofen, naproxen, Y-5544, diclofenac and aminopyrine to RK13 cell were found to be 130, 290, 200, 140 and greater than 500 mug/ml respectively. On the other hand, ID50 values of each drug to FL-cell were found to be 105, greater than 500, 180, 170 and less than 500 mug/ml respectively. Minimum concentration, cuased by the detachment of the cell from the vessel wall, was as follows: to RK13 were ibuprofen 125 approximately 250, paproxen 250 approximately 500, Y-5554 125 approximately 250, diclofenac 62.5 approximately 125, aminopyrine 1000 approximately 2000 each mug/ml. On the other hand, to FL-cell were ibuprofen 62.5 approximately 125, naproxen 125 approximately 250, Y-5554 62.5 approximately 125, DIClofenac 62.5 approximately and aminopyrine 1000 approximately 2000 each mug/ml respectively.