Hypersensitivity actively induced in utero is shown to persist for a longer period than passive sensitization. The degree of hypersensitivity, its duration, and its transmissibility appear to be influenced by the time elapsing between the original injection of the parent and parturition. A pregnant guinea pig receiving a parenteral injection of antigen 2 to 4 days prior to parturition transmits a state of hypersensitivity to two succeeding generations. The sensitization of the FI generation is due to the passage of antigen. The sensitization of the FII generation is due to the passage of antibodies formed in the FI generation. This prevents any further transfer of the hypersensitive state. Though hypersensitivity occurs in two successive generations, the phenomenon is congenital and not hereditary. We believe that this phenomenon demonstrated in the guinea pig is fundamentally related to the problem of congenital sensitization of the human being.