A rapid and accurate detection of HLA class II antigen is essential for transplantation and for the understanding of disease susceptibility. Recent molecular genetic studies have revealed that the number of class II loci and the number of alleles at these loci are greater than had been previously detected. It is, therefore, of great importance to detect these extensive polymorphisms. A great deal of effort has been made on identification of individual HLA class II specificities at the DNA level, called "DNA typing." What seems to be lacking, however, is handling simplicity. Here we accomplished a simple method for HLA-DRB1 typing based on hybridization of acridinium-ester (AE)-labeled DNA probes to amplified DNA. This method is called hybridization protection assay (HPA). By using 13 AE-labeled probes, 20 homozygous B-cell lines and leukocytes from 80 healthy individuals were typed by HPA. The results were completely consistent with those obtained by polymerase chain reaction--restriction fragment length polymorphism. This method is suitable for mass screening because of its procedural simplicity and swiftness.