Most B lymphocytes recognizing the major epitope on the self Ag mouse cytochrome c (CYT) express one particular VH gene (19.1.2) in combination with one of two Vkappa genes (R9 or 2G5). This restriction made it possible to observe changes in the primary structure of mouse CYT-specific mAb as the B cell response to mouse CYT (coupled to OVA) progressed. Thus, somatic mutations at positions 31 and 58 in the VH gene complementarity-determining regions (CDR) 1 and 2, respectively, and tyrosine at position 96 in the Vkappa-Jkappa join were frequently selected during the response. The affinity constant (ka), which increased as much as 90-fold due to a decrease in the off-rate constant, correlated with the number of somatic mutations but not strictly with the commonly selected changes. There appears to be little selection for a particular H chain CDR3 in the mouse CYT-specific mAb since it is extremely variable, in sequence and length, throughout the response. This variability may reflect that mouse CYT is a self Ag. Otherwise, the Ab response to this protein Ag is consistent with the paradigm for affinity maturation that is well established for Ab responses to haptens.