Delayed reaction to DNP conjugate prepared in vitro from the extract of homologous epidermis could be clearly detected on intradermal injection in guinea pigs sensitized by epicutaneous application of DNCB and intradermal injection of Freund’s complete adjuvant to the previously applied area, but not to DNP guinea pig serum. The animals developed the most intense delayed reaction to DNP conjugate prepared from the subcellular small granule fraction of homologous epidermis. There was significant difference in the reactivity of this conjugate and DNP-soluble fraction of the epidermis. The animals also showed positive reactions to DNP conjugates prepared from the extracts of homologous heart, liver, kidney and heterologous (human) epidermis. This finding is explainable by cross-reactivity which exists between these conjugates and DNCB conjugated in vivo to autologous epidermi