In blood samples of 88 healthy donors various red cell, HLA-A, -B, -Cw and -Dr antigens were determined by standard methods. Lymphocyte reactivity was measured by 3H-thymidine uptake into DNA of lymphoblastic cells stimulated by 14 different mitogens and antigens. Cluster analysis confirmed by t test and F test defined 3 distinct clusters of lymphocyte reactivity. Cluster 1 in 70% [relative risk (r.r.) 109, p < 0.001] and cluster 3 in 92% (r.r. 33, p < 0.01) showed significantly different cumulations of 3 or more increased characteristic red cell and HLA antigens. Our data suggest that the patterns of both red cell and HLA antigens play a major role in defining immunoreactivity.