Sixteen monoclonal antibodies specific for thymosin alpha 1 [Low et al., J. biol. Chem. 254, 981-986 (1979)] obtained from two fusions with the spleens of three mice [Stähli et al., Meth. Enzym. 92, 26-36 (1982b)] all react with an epitope in the C-terminal half of thymosin alpha 1. Human and foetal calf serum contain substances which cross-react with this epitope. A simple procedure to selectively remove the cross-reactive material and a sensitive RIA are described.