Line immunoelectrophoresis of bovine serum revealed a minimum of 37 bovine serum proteins. In line-absorption immunoelectrophoresis, 13 of these proteins were identified by their cross-reactivity with monospecific antisera against human serum proteins. One additional protein was identified by monospecific antisera against bovine immunoglobulins. Crossed immunoelectrophoresis revealed a minimum of 40 bovine proteins and demonstrated their electrophoretic mobility. In crossed-line immunoelectrophoresis all 37 line precipitates were related to their corresponding peak precipitates and the electrophoretic mobility of the 14 proteins identificd was determined.