Whether germinal centers (GC) with follicular dendritic cell (FDC) clusters are the essential sites for affinity maturation of immunoglobulin is still controversial. To re‐evaluate the role of GC / FDC in affinity maturation and somatic mutation in a defined antigen system, lymphotoxin‐α^{– / –} and TNF receptor I^{– / –} mice, lacking GC / FDC, were immunized with (4‐hydroxy‐3‐nitrophenyl) acetyl‐sheep RBC (NP‐SRBC). In contrast to soluble hapten‐carrier systems, NP‐SRBC allows us to compare affinity maturation in the presence or absence of adjuvant. These mice showed a dramatically impaired ability to generate high‐affinity IgG to NP, but retained the ability to produce low‐affinity anti‐NP IgG when NP‐SRBC was used in the absence of adjuvant. In contrast to wild‐type mice, somatic mutation of the expressed IgG heavy chain gene was rarely detected in these GC / FDC‐deficient mice. This suggests that GC / FDC are essential for affinity maturation. Trapping antigen‐specific B cells inside the T cell zone of TNFRI^{– / –} mice may prolong the interaction between T and B cells, which allows class switching but no further affinity maturation of IgG. Interestingly, GC / FDC‐deficient mice could be induced to generate high‐affinity, somatically mutated IgG antibodies by immunization with the same amount of NP‐SRBC antigen emulsified in incomplete Freund's adjuvant or repeated immunization with the antigen alone. Thus, these data support a model in which prolonged availability of antigen is required for somatic mutation and affinity maturation, and FDC or adjuvants facilitate such processes by slowly releasing antigens.