AbstractSalts of formic, acetic and propionic acids and glutaraldehyde were subjected to an assay with representatives of the major groups of silage micro‐organisms at pH 4·0, 4·5, 5·0 and 6·0. The antimicrobial spectra of the salts suggested that they are most effective against the least desirable components of the bacterial flora of silage, coliforms, Clostridia and bacilli, and would create conditions for a desirable fermentation in practice. In many instances the activity of the salts was better than the corresponding free acid and it was postulated that, if supplemented with acid, aerobic stability of silage might also be improved. Glutaraldehyde, whilst being less selective as an antimicrobial agent, possesses properties which would encourage a lactic‐acid fermentation in