The L chains of rabbit antibodies directed against group a allotypes exhibited in many instances a high degree of homogeneity as measured by L chain banding patterns in alkaline urea polyacrylamide gels. Amino terminal sequence analyses were carried out on six L chain preparations from antibodies isolated from individual antisera or from pools of antisera. The same major amino terminal sequence, Ala-Val-Val-Met, was observed for each of these preparations indication that anti-allotype antibodies preferentially select L chains from a single subgroup. The antiallotype anitbodies were isolated from antisera by elution from IgG immunoadsorbent columns in yields ranging from 0.3 to 1 mg/ml antibody. The specificity of the isolated antibodies was demonstrated by radioimmune assays. Certain fractions were contaminated with a protein that had properties similar to rabbit serum albumin. This contamination was minimized by preadsorption of the antisera. The antibodies were primarily of the IgG class as shown by immunoelectrophoresis and by m.w. of the H and L chains on SDS gels.