Transforming growth factors beta (TGF beta) are multifunctional polypeptides that participate in regulation of growth, differentiation and function of many cell types. The mature TGF beta molecule is a 25 kDa protein composed of two 12.5 kDa monomers linked by disulfide bonds. Human glioblastoma cells secrete biologically active TGF beta 2. Here we report that in addition to the free form of TGF beta 2, a stable complex between a approximately 110 kDa binding protein and TGF beta 2 was isolated from glioblastoma cell supernatant. This binding protein was purified and was found to show sequence identity to part of the beta amyloid precursor protein (beta APP), to be specifically labeled by several different antisera to beta APP, and to be affinity labeled with TGF beta by crosslinking. The complex formation between TGF beta and beta APP may have important implications in regulation of biological activity of the two proteins and in delivery or clearance of TGF beta and beta APP in the brain and other compartments.