A continuous canine lymphoma cell line (DL. 24N) was established by serial transplantation from a diffuse large cell lymphoma into athymic nude mice, yielding subcutaneous tumours at the injection site associated with lymph node metastasis. The transplanted tumour cells, of canine origin as assessed by the karyotype, appear comparable to the large non-cleaved and immunoblastic human cells, as did the initial tumour cells. Immunological studies show reactivity with antibodies to dog immunoglobulins, DT-2 and Thy-1, to MHC-class II antigens, and to both CD10 (CALLA) and CD21 (CR2) human antigens. Such a cell line, showing similarities with some of the human diffuse large cell lymphomas, will thus provide a new tool for further comparative studies of malignant lymphomas.