The antithrombin III (ATIII) isoform pattern of a number of serum and plasma samples was analysed by isoelectric focusing and immuno-blotting. A novel ATIII isoform pattern which was observed in 4/80 children with acute lymphatic leukaemia (ALL) and in 1/4 children with Ewing's sarcoma, has been shown by family studies to be due to a mutant form of ATIII (AT Dublin) in the heterozygous state. The coagulation properties of AT Dublin heterozygotes were normal. In addition the immunological and activity levels of their ATIII were normal. The effects of thrombin and heparin on the mutant ATIII were similar to controls. Neuraminidase treatment reduced the ATIII isoforms to one in controls and two in the mutant. Two-dimensional gel analysis showed the mutant ATIII to have an identical molecular size distribution to the normal form. This mutant is, thus, most likely due to an amino acid substitution giving a more basic molecule that is clinically silent (at the coagulation level). It may be of interest that the frequency of AT Dublin in the ALL group is significantly higher than in the control group (3/430) studied (P less than 0.001).