Polyclonal intravenous IgG (IVIG) was administered as an infusion 6 times every 3 weeks (week 0, 3, 6, 9, 12, 15) in doses of 0.1, 0.4 and 0.8 g/kg BW to determine the dose causing an increase in 12 pneumococcal antibody types above the protective level of 200 ng/ml of antibody N. The dose of 0.4 g/kg BW was found to be optimal in patients with chronic lymphocytic leukaemia (CLL). From the first infusion onwards at least 80% of CLL patients had increases in all 12 antibodies. Five weeks after the last infusion the antibody levels were still elevated in 80% of patients with CLL. The dose of 0.8 g/kg raised all 12 antibodies in 53-73% of CLL patients when assessments were made after each infusion. In multiple myeloma (MM) patients, 73-82% and 73-91% of patients had increased antibody levels, respectively, before and after the 4th-6th infusions at the 0.8 g/kg dose level. However, in only 45-50% of patients did the antibodies remain increased 2 weeks after the treatment at this dose. The dose of 0.4 g/kg caused antibody increases in only 30-50% of patients when measured before the 4th-6th infusion. Serum IgG increased significantly only in the CLL patients, whereas in the MM patients it was high from the beginning owing to the disease. Therefore, the pneumococcal antibody levels were a better marker for the purpose of dose finding. The dosage recommendation in CLL is 0.4 g/kg every 3 weeks until week 12, when steady state is reached. The maintenance dose is 0.4 g/kg every 5 weeks. In MM patients, who have a faster elimination rate of antibodies, the recommended loading dose is 0.8 g/kg, followed by 0.4 g/kg every week as a continuous treatment. Treatment with IVIG in CLL and MM was generally well tolerated. Only 25% of patients experienced minor side-effects, the most frequent being febrile reactions, shivering and headache.