Aim of the study was the establishment of a standardized in vitro culture system for studies on proliferation and differentiation of human leukemic pre - B cells. Coincubation with human stromal cells led to a significantly higher proliferation of the cytokine - sensitive leukemic pre - B cell line BLIN-1. Clones from the murine fibroblast cell line L929 provided identical results. Coincubation with the murine cells also resulted in a significantly higher numbers of viable cells in 5 of 8 patient samples with newly diagnosed B lineage ALL. The results show that in vitro bone marrow stromal cells can be substituted by murine fibroblasts and form the basis for a simpler and more reproducible assay system.