Abstract Bacterial hemagglutinins of a pyocyanin-producing strain of Pseudomonas aeruginosa were shown to combine with horseradish peroxidase, as do certain phytohemagglutinins such as concanavalin A. This property could serve for labelling these hemagglutinins by means of peroxidase, which may be stained by H 2O 2 and benzidine, as well as for the attachment of peroxidase to a Sepharose column. The linkage of peroxidase to Sepharose, mediated by the hemagglutinins, was highest when employing bacterial preparations which exhibited mannose-binding activity in addition to the major galactose-specific activity. This linkage was strongly inhibited by 0.05 M d-mannose and 0.5% EDTA, weakly inhibited by 0.3 M d-galactose and not inhibited by d-glucose at the same concentration. The inhibition by EDTA was reversed by addition of 1 · 10 −3 M MnCl 2.