During a recent outbreak of foodborne listeriosis which occurred in France in 1992, investigations were carried out in order to identify the plants associated with the production of contaminated products. These investigations were made in six 'suspect' delicatessen plants following the first epidemiological investigations, and in one 'control plant'. The first visits were made during working operations. Two hundred and seventy samples were taken ('environmental' swabs, air samples, products), with 68% of the swabs being positive for Listeria monocytogenes in raw product areas, and 33% positives in the finished product area. The epidemic phagovar was identified in a single plant preparing pork tongues in aspic. The major causes of contamination identified were contact of cooked products with soiled surfaces, cross-contamination between 'raw' and 'cooked' channels and the inadequacy of cleaning and disinfection procedures. A second visit was also made to five plants to provide validation of their cleaning and disinfection procedures. Among 112 swabs collected, 17% of the samples from raw product surfaces and 7% from finished product surfaces were found to be positive. These results suggest that cleaning and disinfection procedures were unable to eliminate sources of L. monocytogenes when not correctly applied.