Immunoglobulin K genes are constructed during lymphocyte differentiation by the joining of two DNA elements, V^K and J^K, to form both a V^KJ^K coding unit and a reciprocal recombination product. The two products formed in single V^K-to-J^K joining events can be directly isolated through the use of a retrovirally introduced recombination substrate. The structural analysis of a number of recombinants and the derivation of secondary recombination products define some of the basic features of the mechanism of immunoglobulin gene assembly.