W e report a detailed analysis of a B cell defect affecting a patient girl born from first cousin parents, characterized by a severe non-X-linked agammaglobulinemia with a total absence of CD19- cells in the periphery. In the bone marrow, CD19 expression was also highly impaired, resulting in the absence of both B and preB compartments. By contrast, CD34+CD10+, CD34psiL+, and some CD19+CD10+ mostly CD34+ early proB cells were present, although diminished. Semiquantitative RT-PCR analysis performed on mononuclear bone marrow cells indicated that lambda-like, VpreB, Rag-1, Rag-2, and TdT transcripts expressed during proB cell stages were found at normal levels whereas E2A, CD10, Syk, Pax-5, CD19, Igalpha, Igbeta, VH-Cmu, and Vkappa-Ckappa transcripts characteristic of later stages were severely depressed. This phenotype resembles that of Pax-5 knock-out mice, but since the coding sequence of the patient Pax-5 cDNA was shown to be normal, the defect might rather result from an altered regulation of this gene. All these data indicate that the patient suffers from a new genetic defect that results in an arrest of differentiation within the proB cell compartment, i.e., earlier than X-linked agammaglobulinemia, before the onset of Ig gene rearrangements.