To study the relationship between a bovine serum mannan-binding protein (MBP) and a serum protein reactive with a Ra chemotype strain of Salmonella serovar Typhimurium (Ra-reactive factor, RaRF), both proteins were isolated by use of their affinity for yeast mannan and the Salmonella cells followed by affinity chromatography on mannobiose-Sepharose 4B. Both purified proteins showed a major protein band with a molecular weight of 33,000 and a few faint bands in sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis under reducing conditions. In Western blotting with rabbit anti-bovine MBP antibody, the major subunit of both proteins were found to be immunologically identical. Similar findings were also obtained with purified human MBP and RaRF. From these findings, bovine and human serum MBP are suggested to be electrophoretically and immunologically the same as their corresponding RaRF.