The effect of varying the dose or the frequency or of both dose and frequency of immunoglobulin administration was evaluated in 17 patients with primary hypogammaglobulinaemia treated with Scottish National Blood Transfusion Service intravenous immunoglobulin (SNBTS IV IgG), an intravenous immunoglobulin preparation manufactured by the pH 4/mild pepsin method. Individualised dosage of SNBTS IV IgG resulted in trough serum IgG levels of greater than 4.0 g/l in 13 of the 17 patients studied. 6 of the 17 patients had trough serum IgG levels in the normal range (5.0 g/l). In 2 patients, more frequent infusions were required to achieve normal trough serum IgG levels because of their clinical conditions. In 1 of these 2 patients, the frequency could be reduced to three weekly when the gastrointestinal loss of IgG was reduced. There was a low incidence of adverse reactions (6.4%) which occurred mainly during the initial infusions. Most patients benefited from an increased serum IgG level, but the frequency of diarrhoea was unaffected. One patient with severe bronchiectasis continued to suffer from respiratory infections despite normal serum IgG levels.