Erythroleukaemia, until now defined by morphological criteria, has been observed in mice infected with several distinct virological entities, including the polycythaemia-inducing strains of Friend virus complex (FV-P), the anaemia-inducing strains of Friend and Rauscher virus complex (FV-A and RV-A, respectively) and various helper-independent virus isolates derived from the FV-A or RV-A complexes. Whereas erythroleukaemia develops rapidly (within 2-3 weeks) in mice infected with any strain of FV or RV, the helper-independent virus alone is pathogenic in mice only if they are infected neonatally. We now describe how two biological markers can be used to distinguish among the otherwise confusing array of virus-induced erythroleukaemias in mice. The evidence suggests that there are three different classes of this disease: (1) those (S+E+) from which both defective spleen focus-forming virus (SFFV) and erythropoietin-independent, erythroid colony-forming units (CFU-EI) can be recovered, (2) those (S+E-) from which only SFFV (but not CFU-EI) can be recovered, and (3) those (S-E-) from which neither SFFV nor CFU-EI can be recovered. There is a consistent association between the type of virus used to induce the erythroleukaemia and the class of the disease induced.