Four-year hematologic studies were carried out with a total of 51 spontaneously and experimentally infected pigs that developed colibacteriosis. In both cases there were hypochromic anemia, lymphocytosis, and basophilia, and in some of the animals there were hypersegmentation of the nucleus with toxic granulocytes. There were also differences concerning the hematologic indices with the use of individual Escherichia coli strains. They were most pronounced in pigs that were infected with strain O149 and strain 1257 (Poland). Sows that were treated with a killed colibacteriosis vaccine showed no changes in the hematologic profile.