Two types of receptors for insulin-like growth factors (IGFs) were characterized in glioma cell lines established from different human brain tumors of glial origin (astrocytoma grades III and IV) by competitive binding assay, affinity labeling, and protein phosphorylation. Type I IGF receptor is a heterodimer composed of α-subunits (M_r 130,000), which bind IGF I and II with equal affinity, and of β-subunits (M_r 98,000), which show tyrosine kinase activity and autophosphorylation stimulated by IGF I and II with equal potency. The type II IGF binding site is a monomer (M_r 250,000) which binds IGF II with 10 times higher affinity than IGF I. The cellular concentration of type II IGF binding site is about 2- to 5-fold higher than the amount of type I IGF receptor. The characteristics of the two types of IGF receptors in human glioma cell lines are similar to those described recently in fetal rat astrocytes. In contrast the type I IGF receptor in glioma cells is different from that studied previously in normal adult brain regarding the equal affinity for IGF I and II, and the higher molecular size of the α-subunit (130,000 versus 115,000). It is suggested that glioma cells may represent a fetal cell type in tumor development of adult human brain. A role of IGF in malignant glioma has not yet been determined, but the presence of IGF receptors is a prerequisite for cellular actions of IGF.