The immunoglobulin (Ig) heavy (H) and light (L) chain gene rearrangements of the high incidence SJL lymphomas (reticulum cell sarcoma, RCS) have been analyzed. Both primary and transplanted RCS show rearrangements of H and kappa L chains, demonstrating that these tumors are of B cell origin. These data are consistent with previous results indicating that these tumors are a mouse model for follicular lymphoma. A long-term transplanted line and the in vitro line derived from it, cRCS-X, have a single rearranged JH-C gamma 2a fragment and one rearranged C alpha gene fragment which does not hybridize with a probe for the JH gene segments. These cell lines also have two rearranged J kappa-C kappa fragments. Primary tumors and early passages are more heterogeneous with respect to Ig gene rearrangements, possibly because more than one B cell clone is present. Although no synthesis of IgG2a, or of any Ig, could be detected by the in vitro cRCS-X cells, these cells contain abundant poly(A)+ RNA that hybridize with gamma 2a and kappa probes as well as lesser amounts of alpha and epsilon RNA. None of these H chain RNA hybridized with probes for the JH gene segments. The epsilon and alpha RNA are the same size as transcripts of germ-line CH genes which have been identified in other systems. However, the gamma 2a RNA are smaller than previously described germ-line C gamma 2a RNA and appear to be transcribed from aberrantly rearranged JH-C gamma 2a genes.