The phagocytic and bactericidal activity of polymorphonuclear leucocytes (PMNL) obtained from 50 non-splenectomised patients with homozygous beta-thalassaemia was studied. Polymorphonuclear leucocytes suspended in the serum of patients from whom they were derived ingested fewer Escherichia coli, 72.2 +/- 21.8 (mean +/- s.d.) per 50 PMNLs compared with 144.5 +/- 36.8 bacteria phagocytosed by PMNLs from healthy volunteers and suspended in normal serum (P less than 0.01). Killing of ingested bacteria by the PMNLs from patients was also significantly reduced. These abnormalities are in part serum-associated and are due to the presence of heat-labile inhibitor(s) in the patients' serum. When PMNLs from patients were suspended in patients' heat-inactivated serum, phagocytosis increased to 99.2 +/- 29.2 (P less than 0.01). Similar improvement was noted in PMNL bactericidal activity. These abnormalities provide additional information that helps to explain the increased susceptibility to bacterial infections of patients with homozygous beta-thalassaemia.