Summary. Three mouse tumour cell lines grew continuously in 3 |jM 5-bromoclcoxyuridinc (BUdR), One line (MC-2) produced a rcirovirus and altered in morphology in the presence of BUdR or 5-iododcoxyuridine (lUdR), These etTccls. which could be reversed by growih in normal medium, were similar to those reported for the B-16 mouse melanoma line. The B-16 line used in this study, however, as well as a variety of human cells (six melanoma lines and three fibroblast strains), were much more sensitive to BUdR. 0-03-0