Summary The absorption of proflavine by saline suspensions of two Gram-positiveand two Gram-negative organisms has been measured at pH 7 and 37°C. The absorption per unit dry weight of the organisms decreases in the order ( Cl. welchii )>( Str. faecalis, Bact. lactis aerogenes )>( Bact. coli ), although the differences are not striking. There appears to be no correlation of absorptive power with any single property of the cell such as Gram-staining behaviour, inhibitory concentration of proflavine, phosphorus content, cell volume or cell surface.