Leukemic lymphoblasts after culture retain Ig gene rearrangement in the same configuration determined before culture. Shown is the Southern blot analysis of Ig gene rearrangements in leukemic cells from case no. 3 obtained before culture and after 7 months of culture on stroma. Four clones derived in vitro from single leukemic cells were studied. DNA was digested with the restriction enzymes indicated and probed with a J_H probe. Arrows point to the rearranged bands; G corresponds to the germline band. The KB epidermoid carcinoma cell line, which has Ig genes in germline configuration, was used as a control. The faint germline bands seen in some of the clones after culture are probably derived from residual stromal cells.