We have used classical immunogenetic techniques as well as techniques of molecular biology to study the genetics of rabbit immunoglobulins (Igs) and T cell receptors and the regulated expression of the genes that encode these molecules. We are investigating why rabbits produce only trace amounts of Igs with light chains of the K2 isotype. We have now shown that mRNA encoding such light chains can be detected in splenocytes from rabbits infected with Trypanosoma equiperdum and at 100 to 1,000 fold higher levels in comparable mRNA preparations from Basilea rabbits. Basilea rabbits carry a mutation that results in loss of expression of the normal major Ig light chain type (K1b9). Recent studies revealed a mutation in Jk-Ck intron at the 3' RNA splice acceptor site (Lamoyi and Mage, 1985). In studies to further determine whether this is the explanation for the loss of K1 gene expression, we conducted S1 protection analyses and found small amounts of K1b9 mRNA in splenocytes from Basilea rabbits. using intron and Ck probes we found that the relative proportions of processed and non-processed mRNAs differed greatly in preparations from Basilea compared to normal control rabbits. The results support the conclusion that abnormal and/or inefficient mRNA processing results in the Basilea phenotype (non-expression of K1b9 light chains). We have designed and used an oligonucleotide probe that can distinguish mRNA encoding K2 allotypic forms (bas1 and bas2) that differ by only a single base in the codon for amino acid 204. We are continuing to design and use this and other DNA probes to distinguish rabbit Ig and T cell receptor allotypes.