Although few, if any, Ig molecules bearing light chains of the K2 isotype are produced by our laboratory rabbits, we demonstrated transcription of the K2 gene by S1 protection analyses and light-chain sized mRNA by northern analysis with a K2-specific synthetic oligonucleotide probe. Similarly, although mutant Basilea rabbits produce little or no Igs with light chains of the J1-b9 type, we have detected low levels of b9 RNA. We are investigating whether this is aberrantly spliced message since we have found that the K1-b9 gene in Basilea rabbits has a point mutation in the splice acceptor site of the J-C intron. With similar methods, we have not however, detected mRNA or genomic DNA sequences corresponding to the b5 allotype that has been observed serologically to be produced by b9/b9 cells cultured in vitro with b5 anti-b9 and LPS. Thus alternative explanations for serological observations of latent allotypes must be invoked. We have also shown that synthetic oligonucleotides from the first and third framework regions (FR1 and FR3) of variable regions of heavy chains specifically distinguish mRNAs produced by a1 and a2 rabbits. The prototype FR1 and FR3 sequences may not, however, completely correlate with serologically detectable VHa determinants. A strain of rabbits carrying the parental chromosome from which the Basilea mutant was derived has been developed and shown to have a restriction fragment length polymorphism (RFLP) of the K2 gene also found in Basilea rabbits. This allows linkage studies of the K1 phenotype, K2 RFLP and a newly discovered RFLP of the rabbit T cell receptor chain constant region gene. We have analyzed and predicted the locations of kappa light chain allotypic determinants. Predicted determinants were external, located in or near loops, and fell in two clusters of potentially interacting regions within which several overlapping sets of epitopes could occur. Interaction of anti-K1 antibodies with such epitopes on IgG anti-hapten antibodies abolished hapten-mediated dissociation of the antibody from haptgen-coupled cells.