In an attempt to evaluate the role of gangliosides as receptors for fibronectin, a series of Balb/c 3T3 variant cell lines, with a reduced ability to bind the ganglioside-specific ligand cholera toxin (CT), were examined. Initial characterisation showed that the cell lines displayed a generalised reduction in the synthesis of gangliosides more complex than GM3, but not of cell surface glycoproteins. There was no reduction in the levels of fibronectin found at the surface of the variants as compared with the parental cell line and all were able to spread and form focal contacts on fibronectin-coated substrates. These results suggest that complex glycolipids of the 'ganglio' series are not essential for Balb/c 3T3 cells to interact with fibronectin. The role of gangliosides as receptors for heat-labile toxin of E.coli (H-LT) was investigated using CT as a ganglioside-specific control. Both toxins bound to ganglioside GM1 and to Balb/c 3T3 cell membranes. Binding of 125I-labelled toxins was inhibited by either unlabelled toxin. There was no evidence to suggest that CT or H-LT recognised receptor(s), in Balb/c 3T3 cells, in addition to GM1. In rabbit intestinal brush borders at 0°C, there were more binding sites for H-LT than CT and 125I-H-LT binding could not be inhibited by unlabelled CT. At higher temperatures there was some inhibition of 125I-H-LT binding by CT. In Western blots H-LT recognised proteins co-migrating with the major brush border galacto-proteins. Toxin binding to brush borders from the Wistar strain of rat was similar. One of the 125I-H-LT binding sites may be the sucrase-isomaltase complex, since the toxin bound to brush border fractions enriched for enzyme activity. The data suggest that the major binding sites for H-LT in brush borders are not ganglioside in nature but may be glycoproteins.