Screening for HLA specific antibodies is an important part of laboratory testing for transplantation. The conventional technique for screening is complement dependent lymphocytotoxicity. The development and increasing use of the sensitive flow cytometric crossmatch technique has meant that this conventional screening method is often less sensitive than the final crossmatch. The aim of this study was the development of a flow cytometric screening technique which would be as sensitive as the flow cytometric crossmatch and the investigation of the newly developed ELISA screening method PRA-STAT. These methods were used to investigate antibody production in patients with failed transplants. Flow cytometric analysis of antibody binding to pooled cells was found to be a reliable and sensitive method for the detection of HLA class I and class II specific antibodies. PRA-STAT also detects both class I and class II specific antibodies. Both flow cytometric and PRA-STAT screening methods were shown to be more sensitive than conventional cytotoxic screening. Screening of patients with failed transplants by these sensitive methods showed that the majority of patients produce both HLA class I and class II donor specific antibodies following failure of a primary transplant. Flow cytometric and PRA-STAT screening detected antibody production earlier than cytotoxic screening in some patients. HLA matching was shown to be related to both graft survival and to the levels of antibody produced following graft failure with poorly matched grafts resulting in higher levels of sensitisation than well matched grafts. Patients with detectable post graft failure antibodies had a lower chance of receiving a second transplant and had significantly worse regraft survival than patients with no antibody. The results of the study suggest that HLA class II specific antibodies and repeat class II mismatches may be detrimental in regrafts and this requires further study.