AbstractUsing the newly developed hybrid spheroid system, we have been able to measure sensitivities to adriamycin in two sublines of LoVo cells (a human colon adenocarcinoma line). Hybrid spheroids are composed of coagglomerated LoVo cells, and nonproliferating HeLa“feeder”cells, mimicking a population of minitumors. Spheroids of uniform size, but containing different numbers of LoVo cells, were incubated with various concentrations of adriamycin and after washing, plated for colony formation. Binomial counting statistics were achieved by marking sugace-attached spheroids soon after plating, and scoring colonies one week later. From the fraction of noncolony formers, the average number of clonogens per spheroid (clonogenicity) was calculated using the zero term of the Poisson distribution. The effect of treatment with adriamycin was evaluated from the decrease in clonogenicity in parallel series of hybrid spheroids. Response to treatment was found to be independent of cellular clonogenicity in the control, untreated series. Dose-response curves were obtained for each of the two sublines after treatment for 1.5 h with various concentrations of adriamycin. The difference in the response to adriamycin between the two sublines amounted to a factor of 27 (deduced from D0ratios; these are the same as isosurvival dose ratios).