We have extended the method of one-way stimulation in mixed leukocyte culture tests as previously described to quantitate different degrees of stimulation. To demonstrate that the amount of stimulation is immunogenetically meaningful, siblings and parents in families in whom genotyping on the basis of leukocyte antigen data was possible were tested. The prediction that cells of siblings differing from the responding sibling by both alleles at HL-A, stimulate more than do cells of siblings differing by only one allele, was realized in every case. One exception, with cells of a parent, is discussed. It is stressed that the differences measured here are probably fairly strong ones in the majority of cases, and that lesser differences cannot yet be detected reproducibly.