AbstractDirect evidence was obtained for de novo synthesis of AFP by MCF‐7 human breast cancer cells per se. Synthesis was demonstrated by L‐14C‐leucine and L‐35S‐methionine incorporation into immunochemically isolated AFP, and confirmed by radioimmunodiffusion and radioimmunoelectrophoresis. This information indicates that AFP synthesis is associated with normal and neoplastic cells of several different histotypes, and suggests that AFP detected and measured previously in primary human breast cancer tissue cytosol (Sarcione et al., 1983) also resulted from in situ biosynthesis by breast cancer cells per se rather than uptake of exogenous AFP originating from extracellular sources. Evidence that AFP obtained after treatment of 14C‐leucine radiolabelled MCF‐7 breast cancer cell protein with 0.4 M KC1 contained 2.6 times more radioactivity than did AFP obtained before such salt treatment is interpreted as indicating that two different molecular species of de novo synthesized AFP existed in breast cancer cells: (1) larger amount of non‐immunoreactive AFP which became immunoreactive and measurable after KC1 treatment, and (2) smaller amounts of free immunoreactive AFP. 14C‐radiolabelled AFP obtained before and after treatment of cell protein with 0.4 M KC1 codiffused, comigrated with alpha, electrophoretic mobility and gave an identical radioimmunologic reaction both with each other and with added carrier human cord serum AFP. Furthermore, preliminary studies indicated that radiolabelled non‐immunoreactive AFP could be separated from lower‐molecular‐weight free AFP by chromatography on Sephadex G‐200. Taken together, these findings suggest that synthesized free AFP was bound as a non‐immunoreactive high‐molecular‐weight macromolecular complex rather than being covalently linked. Our current working hypothesis is that most of the de novo synthesized endogenous AFP in MCF‐7 human breast cancer cells was rapidly and reversibly bound by hydrophobic bonding to a specific cytoplasmic AFP‐receptor.