We cloned and sequenced T cell receptor (TCR) alpha and beta chain cDNA from a lambda gt10 library obtained from a murine I-Ak autoreactive helper T cell clone MS202. Two types of cDNA clones for the alpha chain and one for the beta chain were obtained. The two alpha chain transcripts used two different V alpha genes: V alpha 4, joined to J alpha 11.2; and V alpha 5, J alpha TA13. The four V alpha 4 cDNA clones obtained did not have a complete sequences, lacking the leader portion. The V alpha 4 genomic gene segment of MS202 was revealed to contain two exons corresponding to the V alpha 4.MD13 cDNA sequence, and the potential RNA splicing signals between the two exons were intact. Both of the alpha chain cDNA clones showed in-frame rearrangements. Immunoprecipitation of 125I-surface-labeled lysate of MS202 with anti-TCR antiserum and subsequent electrophonetic analyses indicated that only one of the alpha chain polypeptides was expressed on the cell surface. Thus, allelic exclusion of the alpha chain in MS202 is achieved by post-transcriptional regulation rather than rearrangements.