The influence of intravenous gammaglobulin infusions (ivGG) on hemolytic complement function and the concentration of serum C3 and its split product C3dg was studied in 20 children with acute lymphocytic leukemia (ALL) undergoing ivGG prophylaxis. IvGG was infused once monthly over a period of 20 months using two different preparations commercially available. Serum and EDTA-plasma were collected before initiation of ivGG therapy (time 1), after 10 and 20 months (time 2 and 3), before and immediately after the infusions. IvGG was infused in connection with chemotherapy (according to the CoAll 82 protocol). 16 of 60 sera collected prior to infusions contained less than 700 mg/dl IgG. Mean IgG concentrations could be raised to 198 mg/dl (time 1), 219 mg/dl (time 2), and 213 mg/dl (time 3), respectively. -CH 50 prior to infusions was below normal in 15 of 59 sera, afterwards in 25 of 59 sera. AP 50 before (after) infusions was decreased in 29 of 59 (36 of 60) sera, C3 in 18 of 60 (24 of 60) sera. C3dg was slightly elevated in one EDTA-plasma prior to ivGG infusions and in 5 of the plasmas following infusions. IvGG infusions resulted in a significant loss of hemolytic activity of serum complement (p less than 0.01, F-test). The effect was more profound if IgG concentrations before infusions were less than 700 mg/dl, but this was true for only one of the two used ivGG preparations. The long term follow up over two years showed no significant changes of complement functions (F-test), indicating complete recovery of complement function from short term anticomplementary effects.