We studied the absorption of both antigenic and degraded beta-lactoglobulin (beta-LG) from the ileum to the portal blood, in rabbits infected at weaning with the entero-adherent Escherichia coli strain RDEC-1. The infection was characterized by high bacterial excretion from days 7 to 18 postinfection (pi), acute diarrhea for 10 days, and considerable growth retardation. Intestinal absorption of beta-LG was measured at four stages of the infection: early (day 3 pi), peak (day 10 pi), late (day 18 pi), and recovery (day 30 pi). During the 30-day period of infection, age-matched control rabbits exhibited a significant decrease in antigenic and degraded beta-LG absorption. In both control and infected animals, more than 90% of the beta-LG was absorbed by a degrading pathway and the remainder in antigenic form by a minor pathway. RDEC-1 infection significantly raised antigenic beta-LG absorption from days 10 to 30 pi, which delayed the decrease that normally occurs with age. Degraded beta-LG absorption was not modified by the infection, except for a slight increase observed at the recovery stage (day 30 pi). These results suggest that RDEC-1 diarrhea increases absorption of food antigens. The subsequent local or systemic immune responses are not known, but pathologic consequences are possible in susceptible individuals.