Penicilloylated proteins which may be found as impurities in 6-amino-penicillanic acid can be exhaustively digested by pronase to yield amino acids and small peptides. This degradation converts the potent polyvalent antigens into a mixture of mostly monovalent haptens which are much less immunogenic and less capable of eliciting immune reactions in sensitized animals. In order to avoid the contamination of 6-aminopenicillanic acid with a proteolytic enzyme, pronase was converted into a water insoluble form by coupling it with bromoacetyl cellulose. This insoluble derivative of pronase retains its activity and broad specificity. It can be readily removed from the medium upon completion of the impurity degradation, to be used repeatedly in a continuous process. The immunological manifestations associated with penicillins are not completely abolished by removal or degradation of protein impurities. Another important cause for these manifestations appears to be polymeric materials which are formed in penicillins. Such polymeric materials were isolated from ampicillin and shown to be capable of binding spontaneously to proteins (e.g. bovine serum albumin). The protein-polymer conjugates, which are formed under physiological conditions (pH 7.4, 37 °C), were found to be immunogenic and to provoke the formation of polymer-specific antibodies.