Previous studies have shown that the divalent-cation independent aggregation of simian virus 40-transformed 3T3 cells (SV-3T3) is mediated by the interaction of endogenous hyaluronate with binding sites on the cell surface. In the present study, the nature of the interaction of hyaluronate with the binding sites was characterized further by examining both the aggregation of SV-3T3 cells and the binding of exogenously added 3Hhyaluronate. The aggregation of SV-3T3 cells was found to be: (1) optimal at pH 7; (2) only slightly affected by variations in temperature between 0 and 40 degrees C; (3) more potently inhibited by the addition of high molecular weight preparations of hyaluronate than by lower molecular weight preparations; and (4) enhanced by increasing the ionic strength of the medium. With the exception of the effects of temperature, these characteristics correspond closely to those previously described for binding of 3Hhyaluronate. The effect of ionic strength on the binding of 3Hhyaluronate by SV-3T3 cells was found to correspond closely to its effect on lowering the viscosity of hyaluronate. Both effects (i.e. binding and reduction in viscosity) had similar dose-response curves for NaCl and MgCl2. This close correspondence between binding and the reduction of viscosity suggested that the effect of ionic strength on binding was due to a change in the structure of hyaluronate. Lowering the temperature (40 to 0 degrees C) also enhanced the binding of 3Hhyaluronate to SV-3T3 cells. However, this effect was blocked if the cells were first fixed with glutaraldehyde, suggesting that temperature mediates its effect through the binding sites. Additional experiments showed that the binding of 3Hhyaluronate by SV-3T3 cells could be prevented by preincubating the cells with several types of lectins.