A number of stimuli known to induce acid hydrolase secretion from cultured macrophages were examined for their ability to activate C3 via the alternative pathway of the complement system. Loss of haemolytically active C3 was checked in normal and C4-deficient guinea-pig serum. For comparison the interactions of cultured macrophages with other agents well known as potent activators of the alternative pathway of the complement system have been investigated. As judged by their activity in these assays, group A streptococcal cell walls, different carrageenan preparations, dental plaque and Actinomyces viscosus were all capable of initiating the alternative pathway but differed with respect to their potency and their ability to inhibit C3 turnover at high concentrations. Zymosan, some carrageenans, polyanethol sulphonate, and Corynebacterium parvum all induce the release of hydrolytic enzymes from macrophages in culture, even in the absence of serum in the medium. The release is time- and dose-dependent and is not associated with loss of the cytoplasmic enzyme lactate dehydrogenase or any other sign of cell death. The parallelism between the capacity of several agents to activate the complement system via the alternative pathway and to induce inflammatory responses in vivo and selective lysosoma enzyme secretion from cultures of macrophages is discussed.