Five years of experience with cell exchanges between laboratories comprising the Southeastern Organ Procurement Foundation (SEOPF) were analyzed in order to evaluate progress in HLA typing proficiency. The results of the analysis of 15 cell exchanges involving a total of 60 cells indicate that the average detection rate was 95% or greater for HLA-A1, 2, 3,9,10,11,28, and 29; B7, 8, 12, 13, 14,15, 17, 27, and 40. HLA-A locus antigens were detected more often (85.8%) than B locus antigens (80.9%). False negative results were more frequent than false positive antigen assignments. Errors in antigen assignments tended to be nonrandomly distributed for certain antigens belonging to cross-reacting groups but usually were randomly distributed for well defined antigens. During the period of the study, SEOPF laboratories demonstrated improved proficiency in the identification of most HLA-A, B, and C provisional (w) specificities. The results demonstrate the benefits of interlaboratory proficiency testing and indicate that cell exchanges can be carried out successfully on a regional basis.