Polyacrylamide beads were covalently linked to proteins and injected intravenously into normal or immune guinea pigs. The beads trapped in the lung produced very mild foreign body granulomas in nonimmune guinea pigs. In immune guinea pigs, severe granulomas developed which progressed through the several characteristic histologic stages with time. Severe granulomatous reactions developed only upon recognition of carrier determinants of hapten-protein conjugates; thus guinea pigs immune to dinitrophenyl hapten (DNP)-hemocyanin developed characteristic granulomas only upon injection of beads coated with hemocyanin or DNP-hemocyanin, but not with DNP on another unrelated antigen. Granulomatous reactivity was passively transferred into normal animals by lymph node cells but not by serum antibody from sensitized guinea pigs.