Pregnant ewes were vaccinated with partially purified, cell-free K99 antigen isolated from an enteropathogenic strain of Escherichia coli, strain B41 (O101:K99:NM), to induce passive immunity via the colostrum in their offspring against an oral challenge with heterologous "calf-lamb" enteropathogenic strains of E. coli B44. After sucking their dams, lambs were dosed orally with 7X10(10)-2.2 X10(11) organisms within 4--21 h of birth. One group of 10 lambs was dosed with cultures of the mucoid (O9:K30(A),K99:NM) form of strain B44 and another group of 10 lambs with the non-mucoid (O9:K99:NM) form; two groups of four control lambs from unvaccinated dams were similarly challenged. All four control lambs challenged with mucoid B44, loose feaces were detected in only two of the four control lambs and in none of the lambs from vaccinated dams. This suggests that the polysaccharide K antigen may contribute to the virulence of "calf-lamb" enteropathogenic strains that possess the K99 antigen. However, lambs passively immunised with colostrum from dams vaccinated with K99 antigen alone were protected against the production of enteric colibacillosis by oral challenge with EPEC strain B44.