Exponentially growing and confluent cultures of 3T3 cells were X-irradiated, and then the cells were subjected to a variety of conditions and treatments. These included balanced salt solution, conditioned medium, serum-free medium, disturbed (trypsin) and undisturbed growth. The cells were then assayed for survival and giant cell formation by separate methods. Repair of PLD was found to occur with most treatments employed, but the magnitude of repair varied. For growing cells, maximum repair (ratio of <tex-math>$D_{0}\text{'}{\rm s}=0.32$</tex-math>) was found employing conditioned medium (CM). For confluent cells, maximum repair occurred if the cells were allowed to remain in an undisturbed state overlaid with CM. For a 6-hr conditioned medium, undisturbed treatment, the ratio of <tex-math>$D_{0}\text{'}{\rm s}$</tex-math> was 1.49. Under postirradiation treatment conditions similar to those above, a graded response was found for giant cell formation that varied in an inverse manner to the magnitude of PLD repair. In both growing and confluent cell experiments, treatments which allowed for maximum PLD repair produced cell size profiles resembling unirradiated populations, while conditions which did not allow for any measurable PLD repair showed a maximum shift in cell size distribution to giants. Conditions which allowed intermediate repair capabilities had corresponding intermediate-size profile shifts.

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