In a prospective study the incidence of allo-immunization and platelet refractoriness was investigated using a consequently leucocyte-poor blood product regime. Twenty-five previously non-transfused patients with acute leukaemia (11 men, 7 women) or autologous bone marrow transplantation for Hodgkin's or non-Hodgkin's lymphoma (2 men, 5 women) received at least 80 donor units of filtered red cells (filtration within 24 h after donation, leucocyte content 8.5 +/- 3.9 x 10(6)/U) and/or of platelet concentrate (produced by the buffy coat method, leucocyte content: 7.8 +/- 4.2 x 10(6)/U). A 1-hour recovery of 20% in three consecutive transfusions, in the absence of clinical factors known to impair increment, was defined as platelet refractoriness. HLA class I antibody screening with a panel of 60 cells was performed before the first transfusion and after 80 U of blood components. Of 25 patients who entered this study, 6 patients developed platelet refractoriness after a mean of 38 units of blood components (range 26-45 U); all 6 were female with a history of multiple pregnancies. In 19 patients regarded as non-refractory, no HLA antibodies were demonstrated (13 men, 6 women). This study, though limited in size, suggests that the use of blood products containing less than 1 x 10(7) leucocytes/donor unit prevents primary HLA class I immunization and platelet refractoriness.