Exposure of platelet concentrates (PCs) to ultraviolet B radiation (UVB) has been advocated as an alternative method for prevention of the onset of HLA sensitization in recipients. In this study, pooled PCs were irradiated in a Haemonetics UV irradiator (Haemonetics Corp, Braintree, MA) at a dose that did not induce platelet activation. The effect of UVB irradiation on prevention of primary HLA sensitization was evaluated in a prospective controlled clinical study performed in cardiac patients undergoing cardiopulmonary bypass. Patients were treated with filtered red blood cells and a single transfusion of either standard (control group) or UVB-irradiated (UVB group) pooled platelets prepared from 12 donors. Five of 39 patients in the control group and 6 of 62 patients in the UVB group developed allo-antibodies against HLA antigens, which is not significantly different (P = .62). This unexpected finding prompted us to check the efficacy of UVB irradiation. We determined UVB-specific DNA damage in cells by measuring the fluorescence from a labeled specific monoclonal antibody against thymine dimers. With this novel flow cytometer technique, we estimated in UVB-irradiated leukocytes in saline that a mean fluorescence intensity (MFI) of 47 +/- 2 arbitrary units (n = 6) correlated with abolition of alloreactivity in mixed lymphocyte cultures and delayed cell death (within 72 hours). MFI in leukocytes suspended in plasma and exposed to the clinical dose of UVB was sixfold higher (310 +/- 41 arbitrary units) and resulted in early cell death (within 24 hours). We hypothesize that this high level of UVB radiation induces fragmentation of the leukocytes. As a consequence, the poor results of UVB irradiation may be explained by the onset of HLA- alloimmunization induced by soluble donor HLA class I antigens processed and presented by host antigen-presenting cells.