Mice are capable of producing large amounts of antibody against BSA in response to stimulation by the antigen in fluid form or with adjuvant. Fluid BSA also induces paralysis, as judged by the incapacity of the mouse to respond later to immunization. The conditions of treatment which lead to immunization or paralysis have been measured. Two zones of paralysis have been identified, one high in respect of dosage and late in respect of duration of treatment, the other low and early. The high, late zone is entered only after an initial period of immunization has been passed through. An interpretation is offered in terms of (i) concomitant immunization, in which some cells become immunized while others become paralysed, and (ii) a double threshold of paralysis. In accordance with this hypothesis, partially paralysed mice make antibody of normal avidity. The response to other antigens of paralysed mice has also been examined. Suppression of responsiveness could not be found, thus confirming the highly specific nature of paralysis. Upon immunization with a cross-reactive antigen, HSA, an extremely weak antibody to the original paralysing antigen could be detected.