AbstractThe formation of D‐penicillamine‐L‐cysteine mixed disulfide from benzylpenicilloic acid, benzylpenilloic acid and benzylpenicilloyl amide derivatives in L‐cysteine suspensions at pH 7,6 and 37° was studied. The reaction is faster than direct penicilloylation of proteins known to be a route to penicilloyl antigenic determinants. The production of S‐bound penicillamine on protein from penicilloyl compounds must therefore definitely be considered as a potential antigenforming step. The reaction may be partly if not fully blocked by acylation of the thiazolidine nitrogen of penicilloyl compounds. When formylation is applied a considerable reduction of the capacity of penicilloyl antigenic determinants to interact with anti‐penicilloyl antibody is noted. A D‐penicillamine‐specific test antigen was prepared by binding D‐penicillamine via thioether links to a partly succinylated poly‐L‐lysine. Clinical test reactions elicited with this conjugate and with penicilloic acid cannot be well correlated. Penicilloic acid probably detects reactions of undefined specificity in addition to D‐penicillamine‐specific ones.