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. 4 obtained before culture and after 2 months of culture on stroma. The latter cells were derived from a bulk culture and from a culture initiated by a single leukemic cell. DNA obtained from the relapse sample and from a stroma-independent cell line (OP-1) originated from the diagnostic sample were also analyzed. 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 bands. KB epidermoid carcinoma line was used as a control.