Clinical experience has shown that it is often difficult to identify HLA-DR antigens on peripheral blood from long-term dialysis patients, leukemic patients, or blood in poor condition due to age of specimen or delivery conditions. We have found that we can accurately detect HLA-DR antigens on peripheral blood T cells that have been activated with PHA and expanded with commercially available Interleukin 2. An advantage of this technique is that a minimal number of peripheral blood cells is required at the initiation of T-cell activation. The expansion of T cells to a number sufficient for HLA-DR analysis requires 7 to 10 days. To test the validity of this protocol, we analyzed the HLA-DR antigens on a total of 13 normal donors using both conventional techniques and activated T cells. In every case, HLA-DR detection was identical by both methods, and no false positive or negative reactions were observed. Next, we used activated T cells to analyze the HLA-DR antigens in patients that had been difficult or impossible to DR type using conventional methods. We successfully identified HLA-DR antigens on 16 of 16 bone marrow transplant candidates who were previously untypable. We also identified HLA-DR antigens in all of the eight renal dialysis patients who had been untypable on several previous occasions. Subsequent phenotypic analysis of family members for those patients confirmed that the HLA-DR antigens genetically transmitted from family were detected by this method. In summary, we consider that activated T cells can be used effectively and reliably for analysis of HLA-DR antigens for patients who are otherwise difficult to type by conventional methods.