In the following communication, it has been attempted to make a comparison between the opsonic indices obtained from the blood of the insane and those from the blood of normal individuals, and to draw the conclusion that in the case of those patients suffering from an abnormal mental condition we have a slight but nevertheless perceptible lowering of these opsonic indices. As only fifty-two cases have been examined, it is doubtful if one would be justified in drawing any definite conclusion from such small data.