Leukotactic factors derived from the complement sequence consist of C5-related products, including the small cleavage product and the higher molecular weight complex (C567). A leukotactically active cleavage product (of low molecular weight) has also been described. The C5-related factors have been found in two conditions: soluble tissue extracts of immunologic vasculitis and in synovial fluids from patients with rheumatoid arthritis. The C3-related product has been found in synovial fluids from patients with inflammatory, nonrheumatoid arthritis and in extracts of experimentally infarcted myocardium. The presence of these factors may be causally related to the acute inflammatory nature of the cellular exudates.