In this study, the serum sensitivity of 23 P. haemolytica isolates and 18 P. multocida isolates was determined by incubating dilutions of bacteria with equal volumes of fresh or heat-inactivated bovine serum for one, two, or three hours. Clinical isolates of both Pasteurella species were resistant to serum, whereas isolates from asymptomatic cattle varied in serum susceptibility. The classical pathway of complement appeared to be the principal means of complement mediated killing as detected by incubation in the presence or absence of EGTA-MgCl2. Lyzozyme and iron saturation of serum did not greatly affect serum susceptibility with either of the Pasteurella species.