The primary immunoglobulin (Ig) repertoire in the mouse develops during fetal life in the liver. The first Ig gene rearrangement‐‐the joining of a DH to a JH gene segment‐‐contributes largely to the diversity found in CDR3, as well as potentially encoding the D mu protein which is believed to function in the development of a B cell. In this report, the number of DJH joins in two mouse strains, C57BL/6 and BALB/c, were enumerated from days 12 to 16 of fetal development. It was found that the number of DJH structures increased from less than 300 per liver on day 12 to greater than 700,000 (C57BL/6) and 300,000 (BALB/c) on day 16. Each JH gene segment was used approximately equally on each day examined. When the DJH structures were examined by cloning and sequencing it was found that the DJH reading frame (RF) usage (with respect to JH) was not random‐‐RF1 was used 70% of the time. Moreover, a single D gene segment, DFL16.1, was used in greater than 50% of all joins reinforcing the notion that the fetal repertoire is restricted in its antigen binding potential.