High-resolution chromosome preparations from L5178Y TK+/- 3.7.2C mouse lymphoma cells were obtained using acridine orange in the cell harvest procedure. With this technique it is possible to visualize over 500 bands in elongated mouse lymphoma cell chromosomes as compared to the approximately 230 bands visualized in metaphase preparations. High-resolution lymphoma cell chromosomes are described, and chromosome rearrangements carried in the cell line are characterized by ideograms representing the position, number, size, and relative staining intensity of the G-band patterns. Use of elongated chromosomes of mouse lymphoma TK+/- mutants should facilitate analysis of the cytogenetic effects associated with TK+/- ----TK-/- mutagenesis.