1. The virus of vaccinia in so called roller tube cultures of mixed embryonic chick tissue rapidly increases to maximal titre.
2. Under these conditions the quantity of virus in the tissue remains at or near the maximum for at least 9 weeks and considerable amounts are present in the fluids removed each day.
3. The same results are obtained when only fragments of embryonic chick heart are employed.
4. Many, though not necessarily all, of the cells in infected cultures remain alive and retain the capacity to proliferate. The presence of these living cells is essential for the persistence of the virus.
5. No apparent differences in the rate or amount of growth of cells in infected as contrasted with non-infected cultures can be discerned in the gross.
6. It is suggested, but not proved, that virus is continually being produced rather than simply preserved throughout the period.