An extensive in vitro investigation of the host defence system was performed in 11 sibs of a large kindred with unexplained combination of familial hypocalciuric hypercalcaemia (FHH), interstitial lung disease and increased susceptibility to respiratory infections. The impairment of host defence mechanism was most likely related to granulocyte dysfunction. A severe myeloperoxidase deficiency was the most consistent granulocyte defect (P less than 0.001) and it was associated with a relatively diminished chemiluminescence (P less than 0.001). Moreover, a significantly diminished antistaphylococcal phagocytic (P less than 0.001) and killing (P less than 0.001) activity was found which in the absence of any opsonizing defect implicates an intrinsic granulocyte dysfunction. We found no abnormalities in number of B and T lymphocytes nor in the balance between helper and suppressor cells determined with monoclonal antibodies. Despite the recurrent infections no elevations of the immunoglobulin subclasses were found. The relationship between the inherited FHH, interstitial lung disease and susceptibility to respiratory infections remains obscure. It is, however, clear that impairment of the host defence might contribute to a decreased life expectancy in this family.