DNA sequencing has revealed extensive polymorphism within the HLA-DRB and DQB genes of the major histocompatibility complex. At least 43 alleles of the DRB loci and 13 alleles of the DQB1 locus are currently recognized. Identification of these alleles can be performed phenotypically by cellular and serological techniques or genotypically by restriction fragment length polymorphism (RFLP) analysis and sequence-specific oligonucleotide (SSO) probing of DNA amplified by the polymerase chain reaction (PCR). However, each method has its technical limitations so the tissue typing laboratory must therefore choose from the range of available techniques to maximize the efficiency and accuracy of HLA typing. In this paper we describe a scheme for the accurate determination of all the serologically defined DRB and DQB allotypes using a combination of serology, RFLP analysis and PCR-SSO probing. The efficiencies of serology versus RFLP analysis for the DR typing of 800 individuals, and RFLP analysis versus PCR-SSO probing for the DQB typing of 317 individuals, are compared and the merits of each technique discussed. This scheme, which types for both HLA-DRB and DQB with an accuracy approaching 100%, is now routinely employed in all HLA studies of disease and transplantation in our laboratory.