Birds are continuously confronted by a large number of stressors, including pathogens. Despite their variety, all stressors induce an elevation in plasma corticosterone concentration, and consequently increase heterophil to leukocyte (H/L) ratio. In order to evaluate and differentiate effects of endocrine (non-bacterial) and bacterial stress on the proportions and ultrastructural characteristics of chicken leukocytes, a series of experiments were conducted with seven-week old chickens exposed either to dietary corticosterone or to intravenous (i.v.)-injected lipopolysaccharide (LPS). Samples were taken for haematological, endocrine, and electron microscopy examination. Administration of corticosterone and LPS significantly elevated plasma corticosterone concentrations and increased H/L ratios. Electron microscopy observations indicated changes in heterophil size, shape, and granulation, and lymphocyte cytoplasmic characteristics. Immature heterophils were observed in the peripheral blood, suggesting that corticosterone and LPS both stimulate an earlier release of heterophils from bone marrow and enhance their influx into blood circulation. The LPS induced a degenerative morphology and the destruction of lymphocytes, whereas corticosterone appeared to stimulate their redistribution rather than destruction. The results indicate that exposure to corticosterone or LPS similarly increase H/L ratios, but differentially alter the ultrastructure of heterophils and lymphocytes. Elucidation of the mechanisms that cause such changes may play an important role in distinguishing between a nonimmune and immune stress challenge at the molecular level.