We use techniques of classical immunogenetics and of molecular biology to study the genetics of rabbit immunoglobulins (Igs) and T cell receptors (Tcr) and to investigate the regulated expression of Ig and Tcr genes during lymphoid cell development. We reported the thirteenth laboratory observation of a VH-CH recombination (R2M) in the rabbit. All thirteen recombinations have occurred in the male parent. The R2M recombination site maps 3'of the VH genes and 5'of the JH genes as did the sites of three previously characterized recombinations. The R2M site maps 3'of the D1d DH gene which is toward the 3'end of the DH-containing region and is probably in or near stretches of repetitive DNA. There may be two or more "hot-spots of recombination" within or near stretches of repetitive DNA in the DH-containing region of the rabbit. We obtained the complete genomic sequences of the DBeta2 and JBeta2 segments associated with a chimeric CBeta gene present in some rabbits. The rabbit JBeta2 cluster has six functional segments, one pseudogene, and a remnant of another pseudogene between JBeta2.2 and JBeta2.3, equivalent to the one found in man at the same location. The JBeta2.5 gene segment of rabbit has lost the splice signal and is a pseudogene unlike its counterparts in man and mouse. The general organization of the gene segments in the DBeta2-JBeta2 regions of all three species is remarkably conserved over long stretches of DNA sequence; there is higher similarity to human than mouse. Rabbits were bred at the NIH to produce elevated levels of lambda light chains lacking c2l and expressing only c7. These rabbits were shown to produce mRNA and proteins with sequences corresponding to the products of a previously identified genomic lambda light chain gene, Clambda6. The production of c2l is known to be due to expression of Clambda5. The c2l-negative phenotype reflects deletion of a region including Jlambda5. Tbe c7-negative phenotype also appears to result from a deletion.