A peroxyacid antimicrobial concentrate and use composition is provided comprising a C.sub.5 peroxyacid in combination with a C.sub.1 -C.sub.4 peroxyacid, a C.sub.6 -C.sub.18 peroxyacid, or mixtures thereof. The combination of these peracids 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. Sanitizing of substantially fixed, "in-place" processing lines in dairies, breweries, and other food processing operations is one utility of the composition.