Terpentecin at a concentration of 0.78 microgram/ml decreased the number of viable cells of Escherichia coli NIHJ to less than one thousandth the starting number in an hour when added to an exponentially growing culture in a nutrient broth. During this time, the turbidity of the cell suspension kept increasing as fast as the control. Microscopic inspection of the cells exposed to terpentecin under these conditions revealed that the cells were elongated. Terpentecin at a concentration of 6.25 micrograms/ml inhibited incorporation of [14C]thymidine into the acid-insoluble material of cells of E. coli NIHJ by 70% in 30 minutes in contrast to little or no inhibition of the incorporation of [14C]uridine or [14C]leucine. Under similar conditions, terpentecin did not inhibit either membrane transport (uptake) of [14C]thymidine into the cells or the metabolic conversion of the precursor into various cellular acid-soluble components. Terpentecin at a higher concentration (70 micrograms/ml) inhibited by 40% in 30 minutes the incorporation of [methyl-3H]thymidine triphosphate into the DNA fraction of toluene-treated cells of E. coli JE6296 (pol A-). Terpentecin showed stronger antibacterial activities against Bacillus subtilis M45T (rec-) and E. coli BE1121 (rec A-) than against their corresponding wild type strains. However, terpentecin showed no mutagenicity by the Ames test with Salmonella typhimurium strains TA100, TA98, TA92, TA1538, TA1537 and TA1535, and with E. coli WP2 (uvr A). Terpentecin at a lower concentration (0.07 micrograms/ml) inhibited growth in vitro of mouse leukemia L1210 cells by 50%. With the mammalian cells again the incorporation of [14C]thymidine into the acid-insoluble cell material was inhibited more strongly than incorporation of [14C]uridine and [14C]leucine. There was no sign of mutagenicity by the micronucleus test using mice.