Twenty-three canine malignant lymphomas were studied to correlate morphologic and immunologic properties of the neoplastic cells. Morphotologic characterization included histologic classification of cell type and growth pattern, anatomic distribution of lesions, and transmission electron microscopic examination. Parameters examined to indicate B- or T-cell nature of lymphoma cells included demonstration of mitogen responsiveness, cell-surface Ig, spontaneous rosette formation with human red blood cells, and scanning electron microscope (SEM) examination of cell-surface features. Results indicated that the cells from histiocytic lymphomas were lymphocytes rather than histiocytes or macrophages. Most cells from lymphomas examined possessed cell-surface ig, indicating B-cell nature. The cell types represented by the different Ig-positive lymphomas were compatible with maturation arrest at different stages in normal lymphocyte differentiation. For the two most common histiologic cell types, nitogen responsiveness and the presence of cell-surface Ig indicated that diffuse, poorly differentiated lymphocytic cases were biologically heterogenous, whereas nodular histiocytic lymphomas were biologically homogenous. Most canine lymphomas had a multicentric anatomic distribution; however, one thymic and two alimentary forms were observed. Lymphomas with a nodular pattern in lymph nodes had multifocal splenic involvement centered on small arteries, whereas lymphomas with a diffuse pattern in lymph nodes had diffuse involvement of splenic white pulp. The cells of Ig-positive and Ig-negative neoplasms examined by SEM were predominantly of the smooth-cell type.