Ex vivo survival of xenografts cultured under different conditions. Cells from xenograft ALL-3 (A) or ALL-19 (B) were retrieved from cryostorage (day 0) and inoculated onto a monolayer of human bone marrow fibroblasts in AIM V medium (▪), a monolayer of MS-5 cells in QBSF-60/F medium (▴), or directly into RPMI 1640 medium containing 15% FBS, penicillin, streptomycin, and l-glutamine (▾). At daily intervals thereafter, viable cells per well were enumerated by flow cytometry, as described in “Materials and methods,” and results expressed relative to a day 0 control. Overall, the survival of xenograft cells cultured on MS-5 cells was as good (ALL-19; B) or better (ALL-3; A) than xenograft cells on human fibroblasts, whereas cells without coculture rapidly lost viability. Error bars indicate SEM.