Vascular (pulmonary and hepatic blood vessels) and nonvascular (gastro-intestinal, reproductive and respiratory systems) smooth muscle preparations obtained from specifically sensitized animals of numerous species including man contract to specific antigen in vitro (Schultz-Dale phenomenon). This immunopharmacological technique is a useful tool to demonstrate immediate-type hypersensitivity in actively or passively sensitized tissues and may be used to investigate the nature of the principal pharmacological mediators involved in a particular tissue as well as to screen anti-anaphylactic (anti-allergic) drugs. However, much remains to be investigated on the complex mechanisms of action of antigen, desensitization and resensitization (recovery) of anaphylactic responsiveness.