A peroxyacid antimicrobial concentrate and use composition is provided comprising a C.sub.1 to C.sub.4 peroxycarboxylic acid, and a C.sub.6 to C.sub.18 peroxyacid. The combination of these acids produces a synergistic effect, providing a much more potent biocide than can be obtained by using these components separately. Other components can be added to the composition such as hydrotrope coupling agents, stabilizers, etc. An effective antimicrobial use solution is formed at low concentrations when the concentrate composition is diluted with water to a pH in the range of about 2 to 8. Sanitizing of substantially fixed, "in-place" processing lines in dairies, breweries, and other food processing operations is one utility of the composition.