The variability of the immune response of 30 rabbits to the type III and of 40 rabbits to the type VIII pneumococci was studied. Sixty-seven out of 70 animals produced, at one time or another, antibodies of restricted electrophoretic mobility, but no general pattern could be defined. One animal produced a single antibody species throughout the six months of immunization. The homogeneity of this antibody population was evidenced by a unique amino acid sequence of the N-terminal part of the light chain. Only an interplay of several competing and cooperating mechanisms could account for the very diverse nature of the immune response observed in these experiments.