We established and maintained long-term cultures of marrow from normal dogs and dogs with lymphoma or leukemia by single inoculations of mononuclear cell suspensions. Media containing only horse sera (as opposed to horse and fetal calf sera) and catalase (for antioxidative effect) supported improved culture viability, as indicated by increased recovery of progenitor cells (granulocyte-macrophage colony-forming units, CFU-GM) and the release of abundant erythroid cells in the cultures for up to 3 weeks. CFU-GM were maintained for at least 3-4 weeks of culture. Culture appearance, cell counts, and assays of CFU-GM were used to compare the culture kinetics of tumor-involved marrow to normal marrow specimens. Cultures of marrow with extensive tumor involvement tended to be less viable, apparently due to a relative lack of competent progenitors. To investigate whether canine long-term marrow culture provided a purging effect similar to the loss of tumor cells noted in human long-term cultures of marrow from patients with chronic myelogenous leukemia (CML) or acute myelogenous leukemia (AML), we established long-term marrow cultures from 28 dogs with histologically confirmed untreated lymphoma or leukemia. Eleven of these dogs had cytogenetically marked tumor cells in the marrow at the initiation of culture. In six dogs with lymphoma and one dog with acute monocytic leukemia (AMoL) French-American-British classification (FAB) M4 leukemia, we could detect no cytogenetic evidence for persistence of the tumor clones in individually plucked or pooled CFU-GM grown from 3-week-old long-term cultures. In one case of AML (FAB M2), 80% of CFU-GM recovered from long-term cultures at 4 weeks still contained an extra metacentric marker chromosome associated with the continued presence of the leukemic clone in the cultures. Our documentation of a purging effect for some tumors supports the use of this canine model system in the investigation of autologous marrow transplantation with long-term cultured cells for humans with lymphoma and leukemia.