A method using PCR amplification and primer extension with fluorescent oligonucleotides was developed to analyze T-cell repertoires. The sizes of the hypervariable CDR3-like regions of the murine T-cell antigen receptor beta chains were measured for all possible V beta-J beta combinations. This analysis shows that beta chains are distributed into at least 2000 groups, a value that provides a lower limit to their complexity. The CDR3 sizes appear to be dependent on the J beta and especially the V beta segment used and correlates with amino acid sequence motifs in the corresponding CDR1 region. This feature of T-cell receptors is discussed.