1. HLA antigens on platelets can be detected by alloantisera used for typing lymphocytes. 2. There is a general correlation between reactivity of sera against platelets and reactivity against T cells. However, some SLE patients have broadly reactive antibodies to T cells that are much less reactive to platelets. These sera may contain IgG autoantibody to non-HLA T-cell autoantigens. 3. The Pl-FCXM may be both more sensitive than the conventional cytotoxic crossmatching techniques and more discriminating than the T-FCXM in detecting relevant deleterious antibodies. All 11 patients with a negative platelet FXCM had functioning kidneys at 1 month, whereas 7 of 12 patients with a positive Pl-FCXM had nonfunctional kidneys at 1 month (p = 0.003). 4. It is suggested that the Pl-FCXM offers a simple, sensitive, and effective method of crossmatching without the high false positive rate of T-FCXM.