Further enhancement of cellular typing for antigens of the new HLA-linked "SB" gene was undertaken by T-cell expansion and cloning. The PLT reagents which define these antigens could be expanded over 100-fold with interleukin 2 (IL-2), without loss of specificity. Cloning efficiencies in limiting dilution of over 50 percent could be achieved, although only 5 percent of these could be expanded extensively. Detailed analysis was performed with clones from a highly restricted PLT reagent raised between an HLA recombinant donor and his sibling whose only known HLA difference was SB4. The antigens recognized by the 11 independent clones analyzed appeared to segregate in this family with HLA. Although the two SB4-positive haplotypes in the family were essentially indistinguishable by the clones, they detected heterogeneity among unrelated donors matched for SB4 (as defined by bulk PLT reagents). Such heterogeneity did not appear to be due to differences between clones in the kinetics of their responses, but could be explainable by complexity of the SB molecule or by new HLA antigens.