ABSTRACT The aggregation in suspension of freshly trypsinized cultured cells (BHK21) has been studied by microscopic and electronic counting techniques. Comparison of cells grown in suspension with cells grown on glass suggests that the aggregation may not be an artifact of tryptic action, but may reflect an adhesive property of the cell surface which survives the dispersal procedure. The aggregation is very strongly inhibited at low temperature, and resembles in this respect the re-aggregation of cells from trypsinized embryonic tissues. There is no detectable delay in aggregation at 37 °C, and cells pre-incubated at 37 °C do not aggregate at 2 °C. These results are inconsistent with the view that the temperature-dependence of cell aggregation in this system is for resynthesis of surface components. The aggregates are readily redispersed by proteolytic enzymes. The adhesions in the aggregates thus resemble in this respect the adhesions between cells in many tissues. Trypsin and chymotrypsin are of similar effectiveness in dispersing the aggregates, but di-isopropylphosphoryl trypsin is completely ineffective, indicating that the enzyme activity of the proteins is involved in dispersal.