Studies of "potentially lethal damage" in EMT6 mouse tumour cells treated with bleomycin either

Summary.-Studies have been carried out into the effect usually referred to as " repair of potentially lethal damage " following the treatment of cells with bleomycin. In vitro, increased survival was seen with delayed subculture of cells in both exponen-tial phase and plateau phase. It was unimportant whether the medium present during the delay period had been previously used to support cell growth. Exposure of cells growing as a solid tumour in vivo to bleomycin (4 mg/kg), gave a surviving fraction of 2 x 10-3 if assay was carried out at 30 min but a surviving fraction of virtually 100 % if assay was delayed until 6 h. Various possible artefacts have been eliminated as reasons for the observations but doubts are raised regarding the nature of the mechanism involved. AN EFFECT usually referred to as "repair of potentially lethal damage " (PLD) following treatment of cells with bleomy-cin (BLM) has been demonstrated both in vitro (Ray et al., 1973; Barranco, Novak and Humphrey, 1975) and in vivo (Hah