Experimental neonatal colibacillosis, in newborn piglets was attempted using 4 groups of enterotoxigenic Escherichia coli (ETEC) strains, as follows: 1) Two strains from serogroup 0149:K91, both producing thermolabile enterotoxin (LT) and K88 colonization factor; 2) Two strains from serogroup 0101:K30, producing thermostable enterotoxin (STa) and K99 colonization factor; 3) One strain from serogroup 0517:K?, producing thermostable enterotoxin of the STb type and K88 antigen, and 4) One strain from serogroup 08:K?, producing STa enterotoxin and a new colonization factor, named F42. All fourteen piglets inoculated orally with these strains of ETEC developed clinical disease and died up to 42 hours after inoculation, being possible to visualize, by indirect fluorescent antibody technique, in all of them, that colonization of small intestine by the inoculated ETEC had occurred. The production of STa "in vivo", into the gut, by strains from group 2 and 4 was an important factor to prove that experimental colibacillosis did occur. In fact, coprocultures either from the diarrheic stools or from the gut contents revealed a high rate of LT+-K88+ and STa+-K99+ colonies recovery. Though some quantitative differences among the examined materials have been observed, the recovery of STa+-F42+ colonies was lower than in the former groups of ETEC strains. However clinical symptoms, production of STa "in vivo" and colonization of the gut of inoculated piglets proved that F42 antigen is undoubtedly a new colonization factor among ETEC involved in porcine colibacillosis.10416711118