Chlorprophenpyridamine (Chlor-Trimeton) maleate and other antihistaminics have recently been used to decrease the percentage of all kinds of allergic reactions, including that to penicillin. Because of this, we used chlorprophenpyridamine maleate in a patient with subacute bacterial endocarditis who had a severe sensitivity to penicillin, a rare combination and in addition an excellent test for the drug. A penicillin-sensitive patient with a serious illness presents a dilemma in therapy. Only four courses are possible: (a) administration of penicillin regardless of risk, (b) attempted desensitization to penicillin, (c) use of other antibiotics with less likelihood of a successful outcome, or (d) administration of penicillin along with a substance such as an antihistamine or cortisone in an effort to obviate reactions. The following case history of a patient with subacute bacterial endocarditis is illustrative of the failure of alternate c, and the success of alternate d. Desensitization was considered, but a recent