Unstimulated mouse peritoneal macrophages, attached to either glass or plastic substrates, responded to factors generated in serum and plasma by spreading and increasing their apparent surface area up to eightfold. Two distinct and dissociable systems were involved. The first appears related to the distinct and dissociable systems were involved. The first appears related to the contact phase of blood coagulation. It is activated by glass and not plastic surfaces, depleted by kaolin adsorption, and inhibited by soybean trypsin inhibitor. In contrast, a separate complement-dependent system can be generated in kaolin-adsorbed plasma. Activation of the complement system can occur either by the alternate or classical pathways and generates a relatively small effector molecule which is dialyzable. These factors presumably influencing the surface membrane and underlying structures may explain the rapid spreading of activated macrophages observed after both infections and chemical peritoneal inflammatory agents.