Level of matching achieved by conventional typing methods. Patients and potential donors were typed by serological methods for HLA-A and -B and DNA-based methods for HLA-DRB1 and -DQB1. Limiting dilution analysis was performed in the graft-versus-host direction to assess the frequency of host specific CTL precursors. Pairs with a high CTLp frequency (>1:10⁵ PBMC) were judged to be mismatched at the cellular level.