Plasma membrane 5'-nucleotidase (5'NT) activity was assayed in lymph node lymphocytes from seven normal control dogs and in malignant lymphocytes from 25 dogs with lymphoma. The lymphoid tumors were classified according to the NCI Working Formulation into five histologic subtypes. When compared with normal controls significantly lower 5'NT activities were found in the lymphoblastic, small lymphocytic and diffuse large (noncleaved) subtypes while no significant differences were observed in the immunoblastic or small noncleaved groups. In addition, dogs with hypercalcemia or paraproteinemia had decreased 5'NT levels. However, no significant differences were found between prognostic groups or histologic subtypes of the NCI classification. In conclusion, canine lymphoma subtypes had generally decreased 5'NT activities which appeared to reflect the B or T cell lineage, degree of maturation and enzyme activity of the cell of origin.