Eight adult patients with hypoimmunoglobulinaemia were randomly allocated to initiation of low- or high-level IgG-substitution. IgG was administered subcutaneously, at 50 or 150 mg ml-1, 20 ml per infusion, by means of a pocket-portable electric infusion pump. Infusions were given 2 to 4 times weekly for 24 months, with a change of dose regimen after 12 months. The desired plasma IgG levels were reached after a mean lag phase of 3 months (range 1-5 months). The median (and ranges) of the individual mean plasma IgG levels during the ensuing 9-month periods were as follows: high-level period, 6.5 g l-1 (range 6.2-7.8 g l-1); low-level period, 3.2 g l-1 (range 3.0-4.0 g l-1). During the high-level period, compared to the low-level period, there was a significant decrease in the following parameters: 'days in bed at home', 'days missed work' and 'days with fever'. No serious side-effects were observed. It is concluded that a plasma IgG concentration of 6 g l-1 can readily be achieved by subcutaneous IgG substitution, and the prophylactic effect is superior to that obtained with a plasma IgG concentration of 3 g l-1.