1. Through the International Cell Exchange, 4 cells were shipped for blind typing to tissue typing laboratories on a monthly basis for the past 15 years. As many as 288 laboratories worldwide currently participate in this exercise. 2. The frequency of detection of a total of 85 HLA-A,B,C specificities by the laboratories has been determined. Each specificity has been classified as to the reliability of identification. 3. Lymphocytes from 27 donors were sent for retesting in as long as a 13-year interval. This provided an accurate picture of the advances in percent detection by the laboratories. 4. At least 5 variants were found by shipping cells to the laboratories: variants of A9, B21, B5, Bw52, and B40. 5. B cell lines were sent to investigate the HLA-DR antigens. At least 11 specificities had a concordance rate higher than 80% and 5 had a rate of more than 95%. 6. HLA-A,B antibodies sent for antibody characterization were classified by a new mass program for the likely epitope against which the sera were directed. As many as 28 different epitopes were detected by the sera sent in the serum exchange.