the Animal Production Branch, Division of Research Serv ices, NIH, Bethesda, Md. Growth of FBL-3Tumor Cells. The FBL-3 cells were a Friend virus-induced leukemia in B6 mice (10). Inoculation of as few as 1 x 10@cells i.p. produced progressive tumor growth and eventually killed the hosts (progressors). In contrast, s.c. inoculation of up to 5 to 15 x 106 cells pro duced only transient tumor grow...