Changes in cell loss and in the relative number of slowly growing colonies were analysed after irradiation of five human glioma cell lines using a micro-colony assay. The cells were seeded at low density on small palladium covered islands which were separated by agarose barriers. The cell density at seeding was chosen to give, on the average, one cell per island. The size of each island allowed growth to a maximum number of 10-15 cells before confluency was reached. Each palladium island was individually identified in a coordinate system and cell detachment was prevented by a thin overlay of agarose. The number of fast growing colonies was, after irradiation, similar for all the cell lines studied. Large variations were, however, seen in radiation-induced cell loss and in the relative number of slowly growing colonies. Cell lines with a high cell loss tended to produce few slowly growing colonies. The variations in cell loss and number of slowly growing colonies strongly influenced the overall growth of the cell populations and there was no correlation between the decrease in the number of fast growing colonies and the decrease in the overall growth rate directly after irradiation. Further studies of tumour cell heterogeneity, with the micro-colony assay, could perhaps reveal at least some reasons for the varying response often seen between tumours when exposed to therapeutical agents.