The results of field and glasshouse experiments showed that sulphur deficiency increased isoflavone concentrations in clover leaves. Severe deficiency almost doubled the concentration of total isoflavones (formononetin + genistein f biochanin A). Usually formononetin was affected as much as, or more than, either of the other two isoflavones. Four clover strains-Dwalganup, Geraldton, Daliak, and Yarloop-which differed widely in their isoflavone concentrations did not differ significantly in their proportionate increases in isoflavone concentrations due to severe sulphur deficiency. Practical implications of the results are considered briefly.