Early in ontogeny, Ig, TCR-alpha beta, and TCR-gamma delta lack N regions. In addition, Ig and TCR-gamma delta junctions preferentially occur at regions of short sequence homology, thus limiting junctional diversity for these neonatal lymphocyte populations. Here, we analyze the extent of heterogeneity in TCR-beta chain junctions made early in ontogeny. DNA and cDNA from fetal/neonatal thymocytes were amplified by polymerase chain reaction, and the V-D and D-J junctions from these randomly generated sequences were analyzed. The D-J junctions were very heterogeneous, and displayed little evidence of homology-directed recombination. The V beta 8-D and V beta 5-D junctions that we analyzed each had a particular junctional sequence that was created at the site of a two-nucleotide homology, but in each case that sequence only comprised 10 to 17% of the total sequences. This junctional heterogeneity of N region lacking TCR-beta chains can be partially explained by a relative paucity of homologies at the appropriate locations near the coding ends, particularly at the D-J junction, but other factors such as the sequence surrounding the homology may also contribute. Thus, TCR-beta chains have extensive junctional heterogeneity early in ontogeny before N regions begin to be added. Since TCR-alpha beta CDR3 plays a major role in binding antigenic peptides, this junctional heterogeneity is likely to be advantageous for establishing a diverse TCR repertoire. We suggest that the sequences of the coding ends of the TCR-alpha beta have been selected through evolution to avoid the restricted junctional diversity resulting from homology-directed recombination.