51p1 is an allele of V H 1-69 that frequently is expressed by chronic lymphocytic leukemia (CLL) B cells with little or no somatic mutation. The rearranged 51p1 genes expressed by CLL B cells have a distinctive use of D segments D3-3/DXP4 and D3-10/DXP′ 1, a favored use of J H 6, and a longer third complementarity-determining region than the rearranged Ig genes used by CLL B cells that express V H 1 genes other than V H 1-69. We examined the 51p1-encoded Ig expressed by blood B cells of healthy donors. In contrast to the infrequent use of J H 4 by 51p1-expressing CLL (eg, 4%), 36% of the rearranged 51p1 sequences from normal blood B cells used J H 4. Furthermore, the D segment use of the rearranged 51p1 sequences from normal blood B cells was not restricted, but reflected the D segment use of nonselected IgH of normal B cells. Finally, the mean length of the third complementarity-determining region for the 51p1 genes of normal blood B cells was 14.6±4.3 (SD) codons. This is significantly shorter than that noted for 51p1-expressing CLL B cells (18.8±3.2; p< 0.0001, n= 51). This study demonstrates that the 51p1-encoded IgH expressed in CLL are not representative of the 51p1-encoded IgH expressed by normal blood B cells, indicating that CLL B cells express IgH that are distinctive from those found in the normal adult blood B cell repertoire.