Temporary skin reactions to penicillins during the acute stage of infectious mononucleosis.

During the acute stage of infectious mononucleosis (IM), 16 out of 19 patients have reacted to skin tests with ampicillin and/or benzylpenicillin or their derivatives. Two thirds reacted to ampicillin and less than half (10/19) to benzylpenicillin, or their derivatives. No reaction was observed with cloxacillin. The lack of reactivity in 3 of 7 patients and the circumstance that the frequency of reactions with the penicilloylated Escherichia coli proteins makes it likely that the greater reactivity to penicillins and in particular to ampicillin in IM is not due mainly to protein impurities, but chiefly to the antibiotic molecule as such. There was no definite relationship between reactivity and clinical involvement. After the acute stage, disappearance of skin reactivity was noted. Intracutaneous testing with penicillins may be used to monitor the increased cutaneous reactivity during acute IM, and it may be helpful when the acute stage is over to determine when a patient may use these antibiotics without side reactions.