The human immune system offers special advantages for study of the development and evolution of the immune response. A variety of human cell lines are available that are arrested at various stages of development, and human genes provide a convenient evolutionary point of comparison with the already well characterized genes of the mouse. In this paper, we describe the procedure we have used to clone the human kappa chain genes in both germline and rearranged configurations. We have taken advantage of distantly related probes derived from the mouse and nonstringent conditions of hybridization to find the human genes among phage lambda recombinants formed with partially purified genomic restriction fragments of human DNA. In addition to establishing a physical map of the human kappa C and J regions, we have determined the entire sequence of a germline human constant region gene (the Inv3 allele) and two of its J segments, as well as the V/J recombination site of an active human kappa chain gene. For purposes of comparison, we also determined the sequence of the chromosomal mouse constant region gene and its flanking sequences. Although mouse and human sequences have changed extensively during the 70 million years since the two species diverged. significant blocks of homology have been conserved selectively. Some of these have obvious significance in terms of DNA and RNA splicing reactions. By forming heteroduplex structures between mouse and human genes we were able to identify four human J regions that are much more stringently conserved throughout their coding sequences than are the C region genes. In addition, the middle j structure (J3) of the mouse (which is thought to be inactive) appears to be missing from the human genome.