The contact site A glycoprotein is a developmentally regulated cell-surface component expressed during the aggregation stage of Dictyostelium discoideum. This protein has been implicated in the EDTA-stable (Ca2(+)-independent) type of cell adhesion of aggregating cells. The gene coding for the contact site A protein was disrupted by homologous recombination, using a transformation vector that contained a 1.0-kb cDNA fragment as an insert. Transformants that did not express the protein were identified by colony immunoblotting. These transformants produced three truncated contact site A transcripts. One of them was controlled by the original contact site A promoter, as indicated by its strict developmental regulation and cAMP inducibility; the other two transcripts were transcribed from the actin 6 promoter of the vector. When cell adhesion was assayed in the transformants by agitating suspended cells in an agglutinometer, EDTA-stable adhesion was drastically reduced as compared to wild type, confirming that the contact site A glycoprotein acts as a cell-adhesion molecule. However, aggregation of the transformed cells on an agar surface was not remarkably altered. These results suggest that the contact site A glycoprotein is responsible for a 'fast' type of cell adhesion that is essential when aggregating cells are subjected to shear. When cells are not mechanically disturbed, a 'slow' type of adhesion mediated by other molecules is sufficient for their aggregation.