Infectious hematopoietic necrosis (IHN) is a rhabdoviral disease of rainbow trout (Salmo gairdneri). Trout were injected with IHNV, and various hematological and biochemical measurements of clinically ill fish were compared to uninfected controls. Infected fish had reduced corpuscular counts, hemoglobin, and packed cell volume, but normal mean corpuscular volume, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration. The percentage of immature erythrocytes was increased, but the percentage of leukocytes was unchanged. Differential leukocyte counts showed a significant decrease in neutrophils, increase in lymphocytes, but no change in monocytes. Unidentifiable necrobiotic cells were prevelant in blood smears and hematopoietic tissue imprints. Plasma bicarbonate, chloride, calcium, phosphorus, bilirubin, and osmolality were significantly reduced, but plasma glucose and anterior kidney ascorbate were unchanged. Plasma pH increased and the alpha fractions of the serum proteins were altered. No change was found in plasma enzymes, except that a LDH isozyme was significantly increased. The alkali reserve was diminished and alterations in acid-base and fluid balance occurred. Death probably resulted from a severe electrolyte and fluid imbalance caused by renal failure.