Human FN1 / Fibronectin. The reactivity of the antiserum is restricted to fibronectin. In immunoelectrophoresis and radial immunodiffusion, using various antiserum concentrations against human plasma, sera and purified fibronectin a single precipitin line is obtained. Testing in electroimmunodiffusion (Laurell) gives a single precipitation line. No precipitation reaction is obtained with other protein in plasma or serum. Cross-reactivity The antiserum does not cross react with any other human plasma proteins as tested in gel- diffusion techniques. Inter-species cross reactivity is a normal feature of antibodies to mammalian serum proteins, since homologous proteins of different species frequently share antigenic determinants. Cross-reactivity of this antiserum has not been tested in detail, however in double radial immunodiffusion a reaction with cow, cat, dog and monkey have been observed.