Cytochemical staining of haematopoietic cells in blood and/or bone marrow from 20 leukaemic dogs led to modification of a diagnosis based on clinical, haematological and histological findings in seven. Sudan black B and chloroacetate esterase served as granulocytic markers. Peroxidase activity was present in the granulocytes and monocytes of 22 healthy dogs but not the blasts of leukaemic dogs. Alkaline phosphatase-positive staining of granulocytic precursors was a consistent finding in granulocytic and myelomonocytic leukaemia, and alkaline phosphatase-positive lymphoblasts were seen in 38% of lymphocytic leukaemias. Diffuse staining by alpha naphthyl butyrate esterase marked monocytes in both healthy and leukaemic dogs. Cytochemical staining was a valuable aid in the classification of leukaemia.