Continuing study for a second year and further analysis of a double-blind placebo controlled trial, already briefly reported, of injections of tyrosine-adsorbed, glutaraldehyde-modified Dermatophagoides pteronyssinus antigen in fifty-one children with perennial asthma and positive bronchial challenge to the antigen, confirms that the patients receiving the treatment reduced their symptomatic medication more than controls, without deterioration of symptoms. Some became symptom-free, when off all treatment. A double-blind placebo controlled trial of continuing treatment for a second year gave evidence of deterioration when the treatment was stopped. Within the treatment group, the improvement was associated with loss of late (6 hr) reaction to bronchial provocation with the antigen, but was not associated with change of immediate (20 min) reaction in lungs or skin. Those who improved in the placebo group did not lose their late reaction. There was a trend for similar benefit from active treatment in the control group, during the second year, though less than in the original active group, and only one lost his late reaction. Only one of the six children with very severe early onset asthma improved. Local reactions to either active or placebo (tyrosine) were seen in half the patients; these were mild and did not influence the treatment. Systemic symptoms occurred shortly after four active injections and after two placebo injections; only one patient stopped the treatment.