A platelet serotonin release test was performed on blood from 10 patients who had suffered from an IgE-dependent allergic reaction to a neuromuscular blocking drug. The results were compared with those of a leukocyte histamine release test. The upper limit of serotonin release induced by neuromuscular blockers was estimated to be 2.3% in non-allergic patients. The test was positive in six patients and was consistent with histamine release in five. Serotonin release induced by neuromuscular blockers comprised 2.9-25% of total platelet serotonin content. We conclude that serotonin is one of the mediators associated with histamine release during anaphylaxis to neuromuscular blockers. Platelet serotonin release tests may be useful for the investigation of anaphylactic responses to neuromuscular blocking drugs.