Intradermal testing has been used in the diagnosis of acute anaphylactic reactions during anaesthesia in 51 patients. Positive results occurred in 32 pa tients and confirmation was possible in 26. Only in one case did a combination of history and intradermal testing not give a diagnosis. Nine of the negative results were confirmed. Intradermal testing is of no value in the diagnosis of reactions to local anaesthetics or colloids but has great value in determining the cause of reactions due to induction agents or muscle relaxants if performed under controlled conditions. It is safe, and requires no expertise in performance or in terpretation.