The lambda light chain genes and the related loci lambda 5, V(preB)1 and V(preB)2 are selectively expressed in the development of B lymphocytes. The sequences and the order of the gene segments are known, but a precise physical map of the lambda locus has not yet been published. Using the Celera mouse genome database we constructed such a map. In this paper, we confirm the order of the gene segments as being V lambda 2 - V lambda X - J lambda 2 - C lambda 2 - J lambda 4 - C lambda 4 - V lambda 1 - J lambda 3 - C lambda 3 - J lambda 1 - C lambda 1 and give precise germ line distances. We located the V(preB)2 locus 3' to the lambda locus, 1,077,001 bp downstream; the V(preB)1 locus, 2,180,618 bp downstream of the lambda locus; and the lambda 5 gene, 4,667 bp downstream of the V(preB)1 locus. Except for the V(preB)2 locus all other loci are in the same transcriptional orientation.