Colonial morphological variations among enterotoxigenic Escherichia coli which lack K88, K99, and 987P (3P-) were shown to be correlated with expression of several surface antigens, i.e., type 1 pili, F41 pili, and somatic and capsular antigens. Such correlations were established by electron microscopy, serology, and hemagglutination of cells derived from these specific colonial types. Identification of F41 produced by the 3P- enterotoxigenic E. coli strains was made by immunodiffusion in agarose gel, immunofluorescence microscopy, subunit molecular weight determination in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and hemagglutination of F41-piliated 3P- cells and purified F41 pili. Two of the 3P- enterotoxigenic E. coli strains were also shown to be virulent in conventional neonatal pigs and calves. Intense immunofluorescence staining by reference F41 serum of ileal villi of 3P- -infected animals indicated that F41 was expressed during the disease process.