BACKGROUND Inhalation of fumes from heated colophony (pine resin) is a recognised cause of occupational asthma, although the mechanisms by which colophony produces symptoms are unclear and specific immune responses to colophony have not been reported in sensitised workers. A study was carried out to determine whether colophony is antigenic. METHODS The immune responses to colophony were studied in C57BL/6 mice and Dunkin Hartley guinea pigs after intraperitoneal injection of colophony conjugated to bovine serum albumin (BSA) or human IgG by a mixed anhydride procedure. Colophony and dinitrofluorobenzene were also compared in an assay of dermal sensitisation. RESULTS Mice immunised with the colophony conjugates produced antibodies which recognised conjugates of both BSA and human IgG irrespective of which had been used as the immunogen. Solutions of unconjugated colophony inhibited the binding of antibodies to the BSA-colophony and BSA-abietic acid conjugates, confirming that the antibodies recognised one or more components in the colophony. Portuguese colophony also abrogated the antigen binding of serum from guinea pigs immunised with the BSA-colophony conjugate. Spleen cells from immunised mice proliferated in the presence of the conjugates. Although there was some cross reactivity in these responses, it was not as marked as in the antibody assays. Unconjugated colophony failed to induce an immune response when injected intraperitoneally with adjuvant. Skin sensitisation could not be induced in mice by topical application, or by subcutaneous or intradermal injection of unconjugated colophony. CONCLUSIONS Colophony components have the potential to act as haptens and an immune component could be involved in the pathogenesis of occupational asthma in workers exposed to colophony. Colophony is not readily immunogenic unless conjugated ex vivo to proteins.