We previously demonstrated that the α1(l) polypeptide chain of collagen can bind and activate polymorphonuclear neutrophils (PMN). In the present experiments, performed in culture grade 96-well plastic plates coated with collagen, fibronectin, or other proteins, adhesion was assessed by staining the adhering cells after 30 min with crystal violet and measuring absorbance at 560 nm, and activation of PMNs was assessed by measuring the amount of O2- formed. Adhesion occurred at 17 and 37°C but activation at 37°C only. Monoclonal antibody anti-CD 18 inhibited adhesion, showing that the receptor of collagen I on PMNs is a β2 integrin. On the other hand, adhesion of PMNs to fibronectin was inhibited by monoclonal antibodies to CD18 and to CD11b.