IGF binding proteins (IGFBP) regulate the bioavailability and bioactivity of IGF. The major IGFBP in serum is IGFBP-3. We investigated whether sera from children with malignancies show alterations in levels of IGFBP-3 as measured by Western ligand blot analysis (WLB) and RIA with alpha IGFBP-3gl, a specific rabbit polyclonal antibody. Furthermore, IGFBP-3 proteolysis was quantified by densitometric analysis of [125I]IGFBP-3 protease assays, and IGFBP-3 fragments were visualized by Western immunoblot with alpha IGFBP-3gl. We examined sera from 21 children with solid tumors, five patients with sarcoma who had reached complete remission, and 13 children with acute leukemia. Serum samples were collected at diagnosis, before initiation of therapy. Sera of 10 healthy children served as normal controls. Children with solid tumor or leukemia had significantly higher (p < 0.001) IGFBP-3 protease activity in serum than did normal controls or patients with sarcoma in complete remission. Corresponding to this finding, densitometry of WLB showed lower IGFBP-3 levels in sera of children with malignancies in comparison with normal controls. The negative correlation (p < 0.001, r = -0.80) between IGFBP-3 proteolysis, as measured by [125I]IGFBP-3 protease assay, and IGFBP-3 band density on WLB indicates that proteolysis is the probable reason for reduction of IGFBP-3 on WLB. IGFBP-3 concentrations measured by RIA were in the normal range for most patients, further indicating that differences in serum IGFBP-3 levels measured by WLB reflect protease activity.(ABSTRACT TRUNCATED AT 250 WORDS)