A radioimmunoassay for (+)-abscisic acid (ABA) was developed and applied to the analysis of free ABA in extracts of apple (Malus pumila Mill.) and sweet pepper (Capsicum annuum L.) leaves at various stages during extract purification. Conjugates of ABA, were quantified after alkaline hydrolysis. The validity of the radioimmunoassay was tested by comparison of immunoassay estimates of ABA at different levels of extract purity with high-pressure liquid chromatography (HPLC) and combined gas chromatography-mass spectrometry. The antiserum, raised against (+)-ABA, was almost equally sensitive to (-)-ABA. Serum cross-reactivity with the methyl ester of ABA was 160% and with the glycosyl ester of ABA was 34%. Cross-reactivity with protein-ABA conjugates was very slight for C~,-conjugated keyholelimpet haemocyanin, but about 1000% for Caconjugated alkaline phosphatase. Other compounds tested showed extremely low or undetectable cross-reactivities. Further evidence for the specificity of the assay came from the agreement between the results using different assay methods for both apple and pepper extracts, and from the observation that the only zone of immunoreactivity on HPLC elution profiles corresponded with authentic (+)-ABA. The use of polyvinylpyrrolidone in the assay minimised interference by other substances in plant extracts. In pepper, free ABA levels increased rapidly during water stress and re-