Introduction. Lactate dehydrogenase-elevating virus (LDV) was discovered 25 years ago by Dr Vernon Riley † and his colleagues during their work on plasma enzyme levels in tumour-bearing mice (Riley et al., 1960). They found that transplantable tumours of many types caused a five- to ten-fold increase in plasma lactate dehydrogenase (LDH) activity within 3 days of transplantation and before the tumours were clinically obvious. To produce this dramatic increase in plasma enzyme level it was not necessary to transplant cells; cell-free plasma from tumour-bearing mice was equally effective. The raised enzyme level could be serially transmitted from mouse to mouse and proved to be caused by a virus which replicated rapidly in mouse macrophages. Very high titres of viral infectivity (109 ID50/ml) are present in the plasma 24 h after infection, and a stable viraemia at a lower level (104 ID50/ml) is established after 7 to 10 days.