Introduction To define an individual's HLA-A, -B, -Cw, DR and DQ phenotype, the serological technique routinely employed is the microlymphocytotoxicity assay. Target lymphocytes are tested against a panel of HLA specific antibodies each carefully selected to react with one or sometimes two or more HLA specificities. For HLA-A, -B and -Cw (class I) typing a mixed lymphocyte population can be used whereas for HLADR and -DQ (class II) typing a pure B lymphocyte preparation is essential. For successful H L A typing the selection of typing reagents (aUoor monoclonai antibodies) which give reliable positive and negative reactions and which cover as many of the H L A specificities as possible is of crucial importance. The assay entails incubating viable lymphocytes with antibodies under oil in a 60 or 72 well Terasaki tray. Following a 30 minute incubation at 22°C, rabbit serum is added as a source of complement and the test incubated for a further 60 minutes. A positive reaction results in target cell death, which can be assessed microscopically. The most sensitive method for this involves the use of fluorochromes for the differentiation of live and dead cells, although dye exclusion can be measured after staining with eosin or trypan blue. The use of antibody-coated magnetizable microspheres has enabled the reliable separation of pure B lymphocytes for HLA class II typing and can also be applied to the separation of pure T lymphocytes for HL A class I typing. The microlymphocytotoxicity assay can also be used to define HLA specific antibodies by testing sera against a panel of selected target cells from individuals of known HLA phenotypes. It is also the technique applied to the cytotoxic crossmatch which is essential prior to renal transplantation and prior to thoracic organ transplantation in sensitized recipients. Genes within the human major histocompatibility complex (MHC) code for H L A antigens expresse d on the cell surface. HLA class I molecules (HLA-A, -B, -Cw) are universally expressed on all nucleated cells, albeit to varying levels, whilst HLA class II molecules (HLA-DR, -DQ) have a restricted expression on B lymphocytes, monocytes, macrophages and other immunologically activated cells (e.g. activated T lymphocytes). An important feature of the H L A system is the extensive polymorphism of the gene products. The 1991