Feline leukaemia viruses (FeLV) were isolated from cats in Glasgow and New York with lymphosarcoma and from apparently healthy carrier cats. The subgroup composition of each isolate was determined. All isolates contained FeLV of subgroup A(FeLV-A) and a high proportion also contained subgroup B virus (FeLV-B). Virus of subgroup C (FeLV-C) was rare and occurred in association with FeLV-A and, in some isolates, with FeLV-B as well. The same pattern was observed in isolates from British and American cats. The frequency of FeLV subgroups was different in cats with lymphosarcoma and in healthy carrier cats. In cats with lymphosarcoma, 42% had FeLV-A and 58% had FeLV-AB; there was no obvious correlation between virus subgroup and type of disease. In FeLV-positive healthy cats, 65% had FeLV-A and 33% had FeLV-AB. FeLV-C was isolated only from cats with disease. The healthy carrier cats were from multiple-cat households (MCH). Two distinct types of FeLV-infected MCH were found: MCH-A in which the carrier cats yielded only FeLV-A, and MCH-AB in which cats with either FeLV-A or FeLV-AB were present. In MCH-AB half of the cats had FeLV-A and half had FeLV-AB. Overall, the proportion of cats in MCH which were viraemic with FeLV was 42%. However, there was a marked difference in the prevalence of carrier cats in each type of MCH: in MCH-A, 28% were FeLV-positive while in MCH-AB, 53% were viraemic.