The ability of penicillins of varying lipophilicity to induce and elicit cellular allergic responses were analysed in guinea pigs. Epicutaneous application of the penicillins benzylpenicillin (Bp), cloxacillin (Clox) and Bacampicillin® (Bamp) did not cause any unspecific skin irritation. Intracutaneously injected, however, Bamp, Clox and to a lesser extent Bp caused irritation at concentrations of 1.25%. Solutions of 0.12% of the penicillins were inactive in this respect. Cellular allergic responses were induced with Bp, Bamp and Clox after repeated epicutaneous application. The magnitude of responses was related to the lipophilic properties of the penicillins, Bamp being superior. In the guinea pig maximization (GPM) test of Magnusson and Kligman employing intradermal injections of the penicillins with Freund’s complete adjuvant, similar sensitizing abilities of the three penicillins were observed. The cellular allergic responses were elicited with Bp, Bamp, Clox and in addition ampicillin and the 1′-ethoxycarbonyloxyethyl ester of Bp. An extensive cross-reactivity between the penicillins was seen in Bp- and Bamp-sensitized animals, whereas the Clox-sensitized animals showed a specificity limited to Clox. Bamp was shown to possess a superior activity to elicit reactions, possibly due to its lipophilic properties together with an irritating effect exerted by the NH2 g