The effects of the pyrrolopyrimidine antibiotics sangivamycin and toyocamycin on the synthesis of RNA and protein, ribosomal RNA processing, and cell viability were examined in colon carcinoma cell line HT-29. Exposure for 24 hr to toyocamycin caused an exponential type of cell lethality resulting in a 4-log reduction of cell viability, while sangivamycin produced a gradual and self-limiting type of cell lethality resulting in a 1-log reduction of cell viability. Toyocamycin, at a concentration of 1 microM produced total cessation of precursor rRNA processing, while 10 microM sangivamycin produced little or no effect on processing. On the contrary, sangivamycin caused a significant decrease in protein synthesis after 6 hr, while toyocamycin had less effect. The inhibition of protein synthesis by sangivamycin results from an inhibition of the formation of complexes essential to the initiation of protein synthesis. The results suggest that the mechanisms of action of these closely related agents are quite distinct. The marked loss of cell viability caused by toyocamycin correlates with its effect on rRNA processing, while the slow inhibition of protein synthesis appears to be secondary to the loss of ribosome synthesis. On the other hand, the lesser cytotoxicity produced by sangivamycin results from a more direct effect on protein synthesis. Importantly, cells are much less capable of resuming normal proliferative activity after 24 hr of impaired rRNA processing than after a similar interval of reduced protein synthesis.