A study was made of the protective action of the anti-allergic drug ICI 74,917, a phenanthroline derivative, in specific allergen provocation tests and in combating clinical symptoms in thirteen asthmatic out-patients. It was found that ICI 74,917 was able to diminish allergen-induced bronchoconstriction significantly (P less than 0.01) during both the placebo period and during chronic treatment with ICI 74,917 for 4 weeks. The results obtained during these two periods did not differ significantly. There was not, however, any statistically significant difference in the symptom scores or PEF rates during the treatment with ICI 74,917 or placebo for 1 month. This may indicate that although ICI 74,197 is able to prevent bronchoconstriction in an allergen provocation test, it failed to improve asthma clinically. No signs of the phenomenon of tachyphylaxis were observed in the present study. The Student's t-test was used in the statistical analysis.