Various methods for controlling carryover from monoclonal antibodies (MCA) used in the microcytotoxicity assay to detect HLA antigens were investigated by 45 European laboratories. Carryover was demonstrated in most laboratories when using standard techniques (i.e. as for allo-antisera). Taking into consideration factors such as technical ease, time, accuracy and reagent availability, nearly half the participants selected the "no-touch" technique for adding cells and complement as being the best method for controlling carryover in their laboratory. (This method allows the needle of the Hamilton syringe to touch the oil but not the antibody in each well). Most participants (75%) thought that at least one diluent well was still necessary between each MCA well, even when using their preferred technique.