After Ag exposure, somatic hypermutation and subsequent selection play significant roles in shaping the peripheral B cell repertoire. However, the disparate impact of each process has not been completely delineated. To address this, the mutational patterns of a large panel of productive V H DJ H rearrangements of individual human B cells were analyzed and compared with those of a previously reported panel of nonproductive V H DJ H rearrangements. The productive V H rearrangements exhibited a significantly lower mutational frequency and a significantly smaller number of replacement mutations than the nonproductively rearranged genes, suggesting that structural constraints of the Ig molecule and selective influences both impacted the repertoire, militating against replacement mutations. Positive selection favored a mean of four to six replacements in complementarity-determining region 1 (CDR1) and CDR2, and less than two replacements in the framework regions (FRs). In contrast, the negative impact of replacement mutations generated an increased number of silent mutations within both the CDRs and FRs of the productive repertoire accompanied by a net increase in the ratio of replacement to silent mutations in the CDRs compared with that in the FRs. Moreover, there was a negative influence on the distribution of amino acid changes resulting from mutations of highly mutable codons, such as AGY, TAY, GTA, and GCT, preferentially leading to conservative changes in the expressed Ig repertoire. The results are consistent with the conclusion that the expressed repertoire is limited, compared with the potential generated by the mutational machinery, by the dual requirements of avoiding autoreactivity and satisfying structural constraints of an intact Ig molecule.