Molecular analysis of mutations and polymorphisms that are of medical importance requires both accuracy and simplicity. In organ transplantation there is a need for an HLA typing procedure that combines the remarkable accuracy of oligonucleotide genotyping with the simplicity of conventional serological typing. We describe a simple semiautomated method of HLA class II typing consisting of an oligonucleotide hybridisation assay done on microtitre plates followed by automatic colorimetric reading. Individual HLA-DR generic typing for 30 DR specificities, including subtypes of DR1, DR2, DR13, DR14, and DR52, is done on a single plate. The entire typing assay can be completed in less than 4 hours. The procedure has been validated on more than a thousand haplotypes in prospective DR typing of kidney transplant patients, leukaemic patients, and their potential donors. The simplicity of this assay makes it suitable for routine laboratory use. It can be applied to genetic testing in general, including the testing of patients with multiple mutations.