The effects of chloramphenicol (CAP) on the progress of thymineless death (TLD), nalidixic acid (NA) inactivation, ultraviolet (UV) irradiation, and mitomycin C (MC) inactivation were studied in Escherichia coli B, B(s-1), B(s-3), B(s-12), and B/r. This was done before, during, and after inactivation. During the progress of inactivation, it was found that at 10 to 20 mug of CAP per ml, up to 50% of the UV-sensitive bacteria survived TLD and about 10% survived NA. In E. coli B/r, at these concentrations of CAP, about 10 to 15% of the cells survived TLD and about 20 to 25% survived NA. Concentrations of CAP greater than 25 mug/ml actually increased the sensitivity of E. coli B, B(s-1), B(s-3), and B(s-12) to inactivation by either TLD or NA; at 150 mug of CAP per ml, the sensitivity of E. coli B/r to inactivation also increased. When E. coli B cells were incubated in CAP prior to inactivation, the longer the preincubation the longer onset of TLD was delayed; NA inactivation was also affected in that the rate of inactivation after CAP incubation was greatly decreased. Preincubation of E. coli B/r with CAP had much less effect on the progress of inactivation. After thymineless death, incubation in CAP plus thymine led to a rapid and almost complete recovery of E. coli B and B(s-12). Lesser recoveries were observed after inactivation due to UV, NA, or MC inactivation. E. coli B(s-1) and B/r did not recover viability after any mode of inactivation, and E. coli B(s-3) and B(s-12) recovered from UV to about 20% of the initial titer. It was suggested that protein synthesis, in particular proteins involved in deoxyribonucleic synthesis, was a determining factor in these inactivating and recovery events.