Abstract: Strong acid cleaning compositions supplemented with stable, protein-reactive staining dyes are provided which can be used for the acid clean-up of metallic food processing equipment while also giving a visual indication of residual soils on equipment surfaces. The compositions are provided as dilutable concentrates including water, a staining dye and at least about 5% by weight of a strong inorganic acid; the dye is stable in the concentrate under storage at 25.degree. C. for at least about 3 months. The concentrates are diluted in water at the point of use and in such form have a pH of up to about 4. The preferred concentrates of the invention include water, from about 10%-50% by weight of an acid mixture comprising phosphoric, sulfuric and nitric acids, and Acid Violet 19 dye.