Echovirus type I1 was isolated from two cases of Burkitt's tumour and three non-Burkitt turnour cases. Clinical and serological findings indicate that the virus was not aetiologically related to the tumours from which it was isolated, but that it represented a concurrent infection in the patients. The importance of these negative findings in strengthening the candidacy of reovirus type 3 as a possible causative agent of Burkitt's tumour is discussed.