Serious food-borne outbreaks of listeriosis have been reported in North America and in Europe, during the past decade. The predominant risk groups appear to be immunocompromised adults, elderly people, newborn babies and pregnant women. In order to examine the relationship between alimentation, listeriosis and pregnant females, we developed an experimental model using Swiss mice fed ad libitum during 4 days with pellets containing a high concentration of Listeria monocytogenes serovar 4b (10(9) u.f.c/g). Samples were taken from many series of pregnant mice which had been infected respectively by L. monocytogenes from 6th, 10th, 14th and 18th day of pregnancy onwards. This was compared to non infected control series. The transmission of infection from mother to progeny and contamination of surviving progeny were evaluated by Listeria numeration in liver, brain and intestines. Females infected between day 6 and day 10, and between day 10 and day 14 after fertilization, aborted or died of encephalitis. Mice contaminated between day 14 and day 18, were the least prone to experimental listeriosis. On the other hand, some mice contaminated between day 18 and day 22, i.e. at the end of their pregnancy, may develop encephalitis a few days after parturition of a healthy litter. Series contaminated between day 6 and day 10, and between day 10 and day 14 turned out to be highly sensitive to the transmission of infection from mother to young. In two other series (day 14--day 18; day 18--day 22), the young mice contained generally no Listeria. Our experimental model shows the relationship between listeriosis and alimentation. In pregnant mice, sensitivity to infection depends on their gestational status with large individual variability.