SummaryComplete blood counts, differential white blood cell and platelet counts were performed on male and female BB Wistar diabetic rats (BBWd), their nondiabetic siblings (BBWnd) and outbred Wistar rats of the line from which the BB Wistar rats were derived. Most of the observed changes were strain‐related (those present in both BBWd and BBWnd but not in control rats) rather than diabetes‐related (those in BBWd but neither BBWnd nor control rats) and therefore probably due to the inbreeding process. The BBW strain had significantly lower numbers of white cells and platelets, as well as markedly changed differential white cell counts. Differential counts showed a pattern of lympho‐penia, neutrophilia, monocytosis and eosino‐philia. It is possible that these white blood cell changes contribute to the increased susceptibility to infection reported for the BBW strain. No significant difference in serum immunoglobulin concentrations was found in any of these three groups of rats. Therefore, hypogammaglobulinemia cannot account for the increased susceptibility to infections, but it is not possible to rule out an abnormality in the distribution of immunoglobulin fractions as an etiological factor.