Tannins extracted from Cotton Incorporated standard dust (38-75 µm) were removed from solution using serum albumin as a selective precipitant. The technique stoichiometricly removed protein binding tannins from solution. The tannins interfered with antigen-antibody complex formation in the PGE, radioimmunoassay. Histamine release from both E. agglomerant endotoxin stimulated and non-stimulated rat peritoneal mast cells was stimulated by tannin solutions containing one-third or more of the original tannin concentration. PGE(2) production from rat peritoneal macrophages stimulated with the same endotoxin was inhibited by these tannin solutions. Tannins had minimal effect on PGE production by unstimulated macrophages.