An HT 29 cell line derived from human colonic carcinoma was shown to synthesize and release a cobalamin-binding protein. The cobalamin-binding protein was classified as transcobalamin (TC). By gel filtration on Sephacryl S200 HR, we observed that the secreted protein bound to cobalamin had the same size as plasma transcobalamin. Like transcobalamin, the cobalamin-binding protein bound cobalamin but not cobinamide. Purification of the cobalamin-binding protein was performed by heparin-Sepharose affinity chromatography and by Sephacryl S200 gel filtration. The molecular mass of the purified protein was estimated at 44 kDa by SDS/PAGE. The isoelectric point was determined to be 6.4. The purified cobalamin-binding protein reacted with an antiserum produced against human transcobalamin. A 44 kDa band was also identified by SDS/PAGE of an immunoprecipitated homogenate from HT 29 cells labelled with [35S]methionine and in a Western blot of cell homogenates. The secretion of the cobalamin-binding protein was maximal between 10 and 12 days of cell culture and was inhibited by cycloheximide.