The area of highest variability in the antibody-combining site is the third complementarity determining region or CDR3. Based on our preliminary observation of the tremendous variability of this region in the human system we have studied the potential of CDR3 regions for generating diversity in the human B cell repertoire. To this end we generated CDR3-specific cDNA libraries from tissues collected at several stages of human development. Detailed computer analysis of more than 500 sequences reveals that human CDR3 region have the potential to generate more than 10(14) different peptides. The mechanisms responsible for this diversity include rearrangement by inversion, D-D fusion, gene conversion and the frequent utilization of the recently described DIR genes. The specific recombination mechanisms which may explain aberrant rearrangements as well as differences between fetal and adult repertoires are discussed in detail.