A radioimmunoassay procedure is described by which peripheral blood lymphocytes can be typed for HLA-DR specificities. The major advantages of this method are the following: simple and reproducible procedure, no need for B lymphocyte separation, no need for optimal viability, and no need for preabsorption of antisera with platelets. This method will find an application in the genetic and biochemical analysis of the HLA complex, and in the clinical tests of Ia antigens for diagnostic or prognostic purposes and in retrospective transplant studies.