Seven different groups of cats were examined to study the incidence and distribution of feline leukaemia virus (FeLV) in the Netherlands. The indirect fluorescent antibody (IFA) technique was used to detect FeLV antigen. Of the cats with lymphosarcoma (leukaemia), 73.2 per cent and of those with infectious peritonitis, 32.4 per cent were found to be positive for FeLV antigen. Of the sixty-six cats with other tumours, only one, a cat with carcinoma of the mammary gland; was positive for FeLV antigen. Of 557 cats with various lesions, forty-two (7.5 percent) were positive for FeLV antigen. The IFA-test was found to be a useful adjunct in establishing the correct diagnosis. Of all stud cats which had been in contact with FeLV-positive cats, 24.7 percent were positive for FeLV antigen, wheras all those which had not been in contact with these cats, were negative. There was a marked difference between the proportions of FeLV-positive cats in the groups of clinically normal cats which had (20.6 per cent) and which had not (0.4 per cent) been in contact with FeLV-positive cats. Follow-up studies showed that 67.8 percent of the clinically normal, FeLV-positive cats had died from or been sacrificed because of FeLV-associated diseases within twenty months.