To demonstrate the efficacy of a premedication with H1- + H2-receptor antagonists against histamine-release responses in anaesthesia and surgery 3 randomized controlled trials were conducted in patients, volunteers and experimental animals (dogs). Cutaneous anaphylactoid reactions following infusion of polygeline (Haemaccel) in orthopedic patients were successfully abolished by premedication with 0.1 mg/kg dimethpyrindene (Fenistil) and 5 mg/kg cimetidine (Tagamet). Chlorpheniramine (Piriton) was also useful, but dimethpyrindene was more effective in the doses recommended and used. Side-effects of the premedication were not observed when the 2 drugs were slowly administered (2 min each). Systemic anaphylactoid reactions following infusion of polygeline were completely prevented in volunteers by the same premedication (0.1 mg/kg dimethpyrindene and 10 mg/kg cimetidine). Life-threatening reactions could not be tested in human subjects, but were elicited in experimental animals (dogs). In this species which resembles man in its sensitivity against histamine, in plasma histamine levels and in response to polygeline life-threatening reactions were prevented or in especially severe cases diminished to such an extent by the premedication with H1- + H2-blockers that this premedication was finally judged to be very effective against histamine-release responses of any grade of severity. To confirm this clinically very important hypothesis more clinical trials in patients at risk for anaphylactoid reactions to drugs are urgently needed.