WHILE the production of antibodies by explanted tissues or cells from immunized animals is readily demonstrable, most attempts to initiate antibody formation in vitro have met with failure. Only recently has the original claim for success1 been substantiated by the observations of Stevens and McKenna2, who found that normal rabbit spleen produced antibody to bovine γ-globulin after exposure to this antigen in vitro, but only if either the rabbit or the spleen had previously been treated with S. typhi endotoxin.