Applicable to all grades of natural rubber. The principle is based on extraction of any free ricinoleic acid and conversion to the acetate of methyl ricinoleate. Extraction of any castor oil glycerides also present, hydrolysis to ricinoleic acid, and conversion to the corresponding acetate of methyl ricinoleate. Determination of the total acetate by gas chromatography by comparison either with a ricinoleic acid standard, or with ricinoleic acid prepared by hydrolysis of a castor oil standard.