Glutaraldehyde (GA) was tested for its cytotoxic effect on L cell tissue culture system in comparison with formaldehyde (FA). In the first study, the replanted cells were grown to monolayers on the flattened bases of the culture tubes, and then exposed to the drug. In the second study, the drug was added to the cell suspension just put in the culture tubes. In either case, the cells were kept in contact with the drug at 37 degrees C for 24, 48 or 72 hr, after which time the monolayers were removed and the viable cells in each of the tubes were counted up. The change in the number of viable cells was examined in the various concentrations of the drugs and time intervals of cell-drug contact. Both GA and FA showed relatively slight toxic effect when each concentration was 1 microgram/ml. The cells exposed to 1, 10 micrograms/ml of GA or 1 microgram/ml of FA were able to increase in number, though markedly restrained from their multiplication if compared with the control. GA and FA seriously diminished the viable cells at a concentration of 100 micrograms/ml and 10 micrograms/ml, respectively, and they were so toxic that complete cell death was immediately caused even when the concentration of each drug was at 1000 micrograms/ml. Just replanted cells showed less tolerance to the drug effects than the cells of established monolayers; suppression of cell growth was noted with the concentration of 0.8 microgram/ml and above of either GA or FA, and complete cell death was caused by 58 micrograms/ml of GA and 7.0 micrograms/ml of FA.(ABSTRACT TRUNCATED AT 250 WORDS)