The fact that some organisms, viruses, bacteriophages and toxins may be rendered harmless and inactive by the photodynamic action of methylene blue has been well established. Since Perdrau and Todd1 reported that their virus of canine distemper so inactivated possessed a high antigenic property, the antigenic value of these products has become an interesting problem. Rivers2 cautioned workers on the general principle that when the degree of inactivation could not be accurately measured one should be careful in attributing antigenicity to these so-called inactivated agents. Since titrations of minute residual amounts of active virus are still impossible, one naturally turns to bacterial toxins for an answer to this question. Lippert3 reported failure to demonstrate antigenicity in his inactivated tetanus-toxin. Lin4 could not completely detoxify diphtheric toxin. In this note, we present the following findings: Staphylococcal toxin can be completely detoxified by the photo-dynamic action of methylene blue ...