Summary. In 2 clinical studies in 40 conscious human volunteers and 164 orthopedic patients histamine-release responses were diagnosed, defined and classified. Polygeline (Haemaccel) in its now outdated formulation [40] was chosen as a clinical histamine releaser. The main interest was not concentrated on the extreme, the "classical" anaphylactic response, but on the average histamine-release response found in clinical experiments with so many drugs in the last 10 years.In human volunteers 600 ng/kg histamine was i.v. injected. Indicants for a systemic anaphylactoid reaction with the highest incidence ratio were tachycardia, plasma histamine levels > t ng/ml, "metallic taste", flush, congestion of head, "wet eyes" and tears, hypertension and headache. Following polygeline none of these subjects developed a life-threatening reaction, but 12 showed a systemic response, 11 a cutaneous reaction and 17 were non-responders. Indieants for a systemic anaphylactoid reaction with the highest incidence ratio were plasma histamine levels > 1 ng/ml, tachycardia, wheals, sensation of heat, narrowness of throat, hypertension, headache and wet eyes or tears.In a prolective, cohort study in the orthopedic patients 3 subjects with life-threatening reactions, 27 with systemic response, 96 with cutaneous reaction and 38 non-responders were included. Indicants with the highest incidence ratio were tachycardia, plasma histamine levels > 1 ng/ml, erythema and wheals, cough, flush, stuffy nose and facial oedema. With this trial the indicants for diagnosing a systemic histamine release response in volunteers were validated in patients to a large extent.Thus the average histamine-release response was defined by clinical signs such as tachycardia and mild hypertension, scattered hives such as spots of erythema and wheals, respiratory symptoms in the laryngeal and nasal region, such as cough, narrowness in the throat, stuff), nose and sneezing and by pathological plasma histamine levels (> 1 ng/ml), in addition histamine-release responses were differentiated as cutaneous responses, systemic responses and life-threatening responses by clinical and operational criteria and by plasma histamine levels. Using clinical trials and medical * Dedicated to Professor Otto Lindenschmidtt ** Supported by grant of Deutsche Forschungsgemeinschafl (Lo 199/10 and Lo 199/13-6) Offprint requests to: Professor Dr. W. Lorenz (address see above) decision making procedures the incidence of systemic histamine-release responses in patients higher by two orders of magnitude than in other studies reported hitherto.