Abstract The availability of a rapidly growing cell line in tissue culture (Chinese hamster lung, CHL) with high plating efficiency and the ability to form discrete colonies has made it possible to adapt the colony inhibition technique to quantitative studies of complement-mediated lysis of nucleated cells and, in particular, to study the effect of the cell growth cycle on the susceptibility of target cells to immune lysis. During complement-mediated lysis, the cells were examined by phase contrast microscopy, the molecular weight of target cell DNA was estimated by ultracentrifugation in alkaline sucrose gradients and the extent of lysis was determined by a loss of colony-forming ability. Within 8 min after the addition of complement to sensitized CHL cells, morphologic changes in the target cells were noted. Following these changes, an enzymatic degradation of target cell DNA occurred, resulting in a 10-fold reduction in molecular weight of the single-stranded DNA fragments.