A Spanish family was found to have the coexistence of a hereditary haemolytic syndrome associated with excessively leaky RBC membrane to sodium (Na+) and potassium (K+) cations and a partial coagulation factor VII deficiency. Haemolysis was mild in the propositus and the RBC membrane leak included a marked increase in passive permeability to Na+ and K+. This was associated with an increase in active Na+,K(+)-pump activity and in the ouabain-resistant fluxes: Na+, K(+)-cotransport and Na+, Li(+)-countertransport. Factor VII deficiency was of 50% and no clinical expression of the coagulation deficiency was observed. The family study revealed slightly abnormal RBC membrane cationic fluxes only in the father and decreased coagulation factor VII activity of 67% in the mother. Both parents were clinically and haematologically normal. It is suggested that the propositus has inherited the abnormal gene for leaky RBC syndrome from the father and the partial coagulation factor VII deficiency from the mother.