Abstract: An assay to detect the presence of live hepatitis viruses in vitro. Bone marrow cells of leukemic cell line cells are exposed to a body fluid or biological preparation to be tested and the cells are placed in suspension. When using bone marrow cells, growth factors to the bone marrow stem cells are added. It has been determined that presence of a live hepatitis virus suppresses the growth of colonies of the stem cells. Therefore, if the number of colonies growing in the mixture are less than that number present in a culture of cells exposed to a sample that has been determined to contain no live virus, live hepatitis virus is present in the sample tested. The assay is particularly useful to determine the presence of live hepatitis B virus in a vaccine.