The radioallergosorbent test was compared with currently accepted allergy diagnostic techniques in ragweed-sensitive and unselected patients. The radioallergosorbent test was positive in 28 of 30 patients with proved histories, positive scratch tests, and positive nasal provation tests to ragweed. The radioallergosorbent test was also positive in 69 per cent of 40 clinically sensitive patients whose diagnoses were corroborated by scratch tests alone. In unselected patients evaluated by intracutaneous testing with short ragweed extract, 10 minus 3 g per ml, the incidence of positive radioallergosorbent test scores was reduced to 43 per cent. No false positive results were encountered in these studies. A good relationship was observed between radioallergosorbent test scores and the maximal tolerable dose of ragweed administered during a course of "rush" immunotherapy in clinically allergic patients. These results suggest that the radioallergosorbent test has several unique advantages in assessing clinical sensitivity to ragweed.