The variable surface antigens (Vsa) of the murine respiratory pathogen Mycoplasma pulmonis are associated with the virulence of the microorganism in the lung. In strain UAB CT, the antigens consist of an N-terminal region that is combined with one of seven different C-terminal variable regions comprised of tandem repeats. M. pulmonis producing a VsaA protein with about 40 tandem repeats (R40) does not adhere to red blood cells or polystyrene. Strains that produce VsaH contain a short C-terminal region that lacks tandem repeats and adhere to red blood cells and plastic. We isolated and analyzed M. pulmonis strain CT variants (CT182 and derivatives) that produced a VsaA protein with only three tandem repeats (R3). These variants adhered to plastic and red blood cells similarly to the VsaH-producing strain. When the R3-producing CT182 strain or the VsaH-producing strains were incubated with normal guinea pig serum, they were efficiently killed. Killing was abolished when the serum was heat inactivated. In contrast, the M. pulmonis strains that produced VsaA R40 were highly resistant to complement killing. CT182R3 variants that survived the complement killing reactions all produced the R40 form of VsaA and were resistant to complement killing. VsaA R40 is the first mycoplasmal protein shown to be associated with resistance to complement. As both VsaH and VsaA can mediate adherence to plastic, cytadherence, and susceptibility to complement, we propose that Vsa modulates these phenotypes by nonspecific interactions.