LCA, a rapid and efficient computer method to allocate blastogenic responses to positive and negative clusters, is described. The method involves the selection of a partition level for an ordered array of values, such that the resulting two clusters have minimal "within-cluster" variances. Individuals to whom the program assigned the same two HLA-D antigens were MLC-identical (cluster 0) in 50% of the cases. This proportion was lower for HLA-D-identical pairs that typed for Dw4 and/or Dw7, indicating that these antigens are less well defined than the rest. PLTs were less efficient than HTCs in HLA-D typing due to the induction of extra (non-HLA-D determined) responses in the higher cluster.