Abstract Red and green fluorescent polystyrene beads were used to label different populations of cultured human skin fibroblasts. After co-cultivation for 24 h no exchange of label could be observed. Twenty-four hours after fusion the population of red-green heterofluorescent cells was sorted out with a FACS II cell sorter. Microscopical examination of the heterofluorescent population 20 h after sorting indicated, that with Sendai-induced fusion, 50–60% of these viable cells contained more than one nucleus. After polyethylene glycol (PEG)-induced fusion, between 70 and 85% bi- or multinucleated cells were present in the sorted and cultured fraction. Autoradiography using [ 3H]thymidine indicated that the sorted heterofluorescent bikaryons were in fact heterokaryons.