A mouse monoclonal anti-taxol antibody (69E4A8E) and a rabbit polyclonal anti-taxol antiserum were used to measure taxol levels in plant extracts in a double-blind experiment in conjunction with assays by HPLC. 69E4A8E was previously shown by ELISA to be specific for taxol with only a slight cross reaction with another bioactive compound, cephalomannine; the antiserum, on the other hand, was, by radioimmunoassay (RIA), essentially equally reactive with taxol and cephalomannine. Immunoassays of the plant extracts gave results in agreement with that found by HPLC, suggesting that the antibodies can be used in simple routine procedures for the quantification of taxol or taxol-like compounds in extracts of plants or other potential natural sources.