We treated the human monoblastoid cell line, U937, with various cytotoxic proteins or drugs that specifically inhibit protein synthesis and monitored the cells for degradation of chromosomal DNA and other changes. In cells treated with diphtheria toxin (DT), Pseudomonas aeruginosa exotoxin A, ricin toxin, and abrin toxin the chromosomal DNA was degraded into oligonucleosome-sized fragments, the chromatin became condensed, and the cell nuclei fragmented. All of these changes are characteristic of cells undergoing apoptosis, or programmed cell death. Various drugs, including puromycin, cycloheximide, emetine, and anisomycin produced similar changes. An enzymically attenuated mutant of DT, DT-E148S, produced effects identical to those produced by the native toxin, except that a higher concentration of toxin was required, corresponding to the reduction in ADP-ribosylation activity. In all cases, DNA degradation and other changes were observed only after the rate of protein synthesis was reduced to low levels, approximately 10% or less of normal levels. These results imply that inhibition of protein synthesis in U937 cells induces apoptosis, regardless of the mechanism of action of the inhibitor. Differences in the kinetics of induction of apoptosis by the various inhibitors may reflect secondary effects on other aspects of cellular physiology.