Five cases with abnormal sensitivity to respiratory tract infections are described. The cases showed a marked impairment in their cell mediated immunity state. Administration of a chromatographically purified transfer factor component increased the skin test sensitivity to common recall antigens. Interestingly, a similar effect in skin reactivity was observed with repeated skin tests alone, when antigen concentrations, initially high enough to cause a positive reaction, were used. Neither the administration of transfer factor nor skin testing with high antigen concentrations had an effect on blast transformation percentages. The therapy with chromatographically purified transfer factor appeared promising on the clinical condition of the patients.