The protection conferred by egg antibody specific for F18-fimbriae against infection with F18+ Escherichia coli was studied in controlled passive immunization trials involving weaned pigs. Parameters of protection consisted of body weight gain, frequency and severity of diarrhea and recovery of the challenge strain of F18+ E. coli. Weaned pigs at four weeks of age were challenge exposed once daily for three days by oral inoculation with 10(11) cfu of virulent F18+ E. coli followed by daily administration of antibody supplemented feed for 9 days starting from the first challenge day 0. Results showed a dose-dependent response to antibody treatment. The group of pigs given 1:50 titer of antibody in feed had less frequency of diarrhea (P < 0.01-0.05), higher rate of gain (P < 0.01) and lower isolation rate of challenge strain in rectal and intestinal swabs (P < 0.01) compared to non-treated control. In the same manner, the anti-F18 antibody significantly reduced adherence of F18+ E. coli to pig intestinal epithelial cells in vitro (P < 0.01). Results suggest that egg antibodies specific for the F18 fimbriae is a suitable immunotherapeutic agent for pigs infected with F18+ E. coli and that pigs can be protected from overt clinical disease and the subsequent reduced performance arising from infection with this pathogen.