AbstractExtracts of pyrethrum flowers were obtained by treatment with various aqueous and organic solvents. These were tested for immediate and delayed hypersensitivity reactions in guinea pigs previously sensitised to pyrethrum by single subcutaneous injection of the ground flowers suspended in Freund's complete adjuvant. Strong reactions were elicited by various aqueous extracts of the flowers; the allergens are of high molecular weight, being non‐dialysable. A hexane extract (similar to pyrethrum oleoresin) produced a moderate reaction, but a commercially refined pyrethrum extract proved negative.An extract of the spent flowers with methylene chloride showed irritant effects in unsensitised guinea pigs.