Abstract The data for the generation times of V79-1 hamster cells, as obtained by time-lapse photomicrography, suggests an interaction between neighbouring cells. The correlation between the generation times of sister cells was shown to improve progressively with the length of time the sister cells remain in proximity, and an analysis of pairs of non-sister cells where the cells within each pair remained in proximity until one or the other divided showed a correlation if the births of the cells of a pair were separated by not more than 3 hr. More evidence for an interaction was obtained from the growth rates and mitotic indices of cells with different growth characteristics grown in a mixture of one with the other. The cells of one or the other moiety in the mixture were labelled with carbon particles (India ink) or with tritiated thymidine. In the mixture those cells which were delayed in their division by radiation or by being labelled with carbon particles divided sooner and the exponentially growing or less delayed cells were retarded. No interaction occurred across a millipore membrane (pore size 0.22 μ) or between hamster cells and HeLa cells.