A convenient,semi-automated flow injection fluoroimmunoassay method has been developed incorporating the flexibility of sample handling of flow injection system containing a immobilized protein A immunoreactor. Protein A can binds the Fc region of most mammlian antibodies at near neutral pH,and the bound antibodies can be eluted from protein A at acid pH.This peculiar property of protein was exploided to separate the antigen bound to antibody and the unbound ones.Experimental variables have been studied and the method has been used to determine the transferrin contents in human serum.