An investigation into a cluster of Shigella flexneri cases in Michigan and Ohio suggested that consumption of tossed salad at one of several outlets of a restaurant chain was a risk factor for illness. The tossed salad had been prepared at a central commissary and then distributed to the restaurant chain's outlets. Inspection of the commissary revealed poor food handling technique and ill food handlers shortly before the outbreak began. None of the stool samples collected from food workers grew Shigella. None of the salads tested by the Michigan Department of Health grew Shigella. The source of the contamination was suspected to be a food worker at the commissary.