THE ease of evaluation of a therapeutic agent varies in proportion to the natural course of the disease for which it is to be used. In subacute bacterial endocarditis, which, if untreated, carries a nearly uniformly fatal prognosis, therapeutic success is evidenced by an appreciable increase in the survival rate. In lobar pneumonia, the specificity of the sulfonamides and penicillin has been established by a significant reduction in the mortality rate and rapid alterations in the clinical manifestations. The appraisal of therapy, however, in a benign, self-limited disease such as the common cold has been extremely difficult even in well . . .