Colony formation by mock-infected and infected fetal liver cells in methyl-cellulose. Mock-infected and infected fetal liver cells were plated in the indicated conditions in semisolid methyl-cellulose medium and the resultant colonies were scored after 7 days. The data shown are a combination of two separate experiments. In each experiment, duplicate dishes were counted in the case of those containing growth factors, whereas in the case of dishes containing Epo alone or without added growth factors, eight dishes were counted. GFs, growth factors (IL-3, GM-CSF, and Epo).