The expression of insulin receptors and insulin action was studied in cell hybrids and cybrids produced by fusion of the BWIJ mouse hepatoma cell line with nucleated and enucleated mouse L-cells (LEA-2A) respectively. The BWIJ parent and the cybrids expressed high numbers of insulin receptors, whereas the hybrids resembled the L-cell parent with low numbers of receptors. Likewise, the hybrids resembled the LEA-2A cells with high levels of glycogen synthase, whereas the BWIJ cells and cybrids had much lower levels. Both parents, the cybrids, and the hybrids, expressed insulin stimulation of alpha-aminoisobutyric acid influx, but the dose-response curves indicated an increased insulin sensitivity in the cells with the higher receptor concentration. Insulin also stimulated 86Rb+ uptake in the hepatoma parent, hybrids and cybrids, but not in the L-cell parent. These data suggest that insulin receptors, like other hepatoma-specific properties, behave as a 'luxury function' of the hepatoma cell line and are extinguished when the hepatoma cell is fused with a less differentiated cell type. The biological activities associated with insulin action, on the other hand, are much more complex in their expression and probably the result of the interaction of multiple factors that vary in their expression in cell hybrids and cybrids.