In most experiments with filterable viruses designed to show neutralizing or viricidal action by specific immune sera, the possibility of alexin exerting an effect during a preliminary incubation seems largely to have been neglected. It has been assumed that, because such mixtures may be tested only in the animal body for survival or destruction of the virus, and since alexin is there available, that it would be superfluous to add it as a separate component of the mixtures before injection. Although at first view this appears reasonable, certain observations may well cast some doubt on its validity. It has been shown, for example, by Andrewes (1), Bedson (2) and Todd (3) that inactive mixtures of certain viruses with their specific antisera are readily dissociated by physical means (adsorption, dilution or filtration) in such a way that an active virus is again obtained. Moreover, Pentimalli (4) and Duran-Reynals and Murphy (5) have found that the virus of the Rous chicken tumor is readily adsorbed onto or into susceptible cells.