An outbreak of gastroenteritis and fever occurred among persons who attended a picnic associated with a Holstein Cow Show in Elizabeth, Illinois. People had complained about the taste and quality of the milk that was served at the picnic. The milk was cultured and found to be contaminated with Listeria monocytogenes, serotype 1/2b. The same strain of Listeria was isolated from the stool of some of the ill persons and from a tank drain at the dairy. The milk had been pasteurized after the chocolate flavoring had been added. After pasteurization it was pumped into a holding tank that had a breach in its lining. This breach allowed pasteurized milk to leak into the insulating jacket of the tank. As the tank was drained this milk could re-enter the tank through the breach and contaminate the milk. The milk was pumped into sealed half-pint containers and placed in a refrigerated room. They were transported unrefrigerated to the picnic where the milk was served unrefrigerated.