We have developed monoclonal antibodies to most HLA specificities, making it possible for us to devise a simple, rapid, one-step microcytotoxicity test. The test is performed by adding 1 microliter of cells to 1 microliter of antibody-complement mixture predotted on the microtest tray. The reactions are read following a 1-hour incubation period (30 minutes in some instances). The analysis of reactions seen on testing 105 class I antibodies and 50 class II antibodies is shown. A comparison of typing by the standard NIH method and the new one-step procedure showed a > 96% concordance in the 500 T cells and 200 B cells we examined. Class I and class II typing could be performed using B cells, thus obviating the need to isolate both T and B cells for HLA typing.