In 6 different Burkitt lymphoma cell lines with t(2;8) variant translocations (J1, LY66, LY91, BL21, BL64, JBL2) the breakpoints on chromosome 8q+ were mapped in relation to each other and to c-myc by in situ hybridization. The probes used were derived from chromosome 8q24 and comprised a c-myc probe, a probe located 48 kb downstream of c-myc, 3 probes adjacent to the chromosomal breakpoints of BL64, LY91 and JBL2, respectively, and 2 probes located in the 5' and 3' part of the thyroglobulin gene. The breakpoints of LY91 and JBL2 lie less than 200 kb and greater than 200 kb downstream of c-myc, whereas the distance to c-myc of the BL64 breakpoint and of the thyroglobulin probes is unknown. By recording the hybridization signals specific for these probes on chromosomes 2p- and 8q+ of each cell line it was possible to establish the order of breakpoints on band 8q24 relative to the c-myc and thyroglobulin genes as follows: centromere--c-myc--J1--BL64--BL21--LY91--JBL2--+ ++LY66--thyroglobulin--telomere. This information is essential for further mapping of this important chromosomal region.