Four-week-old poults obtained from avian pneumovirus (APV) antibody-free parents were vaccinated with different serial 10-fold dilutions of cell culture - propagated APV vaccine. The birds were vaccinated with 50 μl into each conjunctival space and nostril (total of 200 μl). Each poult of each group was vaccinated in groups that received doses of 4 × 104, 4 × 103, 4 × 102, 4 × 101, or 4 × 100 50% tissue culture infective dose (TCID50), of APV vaccine, respectively. Respiratory signs were seen between 3 and 12 days postvaccination (PV) in the poults that were vaccinated with 4 × 104, 4 × 103, and 4 × 102 TCID50, respectively. In these groups, APV was detected from swabs collected at 5 days PV and seroconversion was detected at 2 wk PV. The groups that were originally vaccinated with 4 × 101 and 4 × 100 TCID50 developed mild clinical signs after vaccination, but neither virus nor antibody was detected PV. At 2 wk PV (6 wk of age), birds from each group, along with five unvaccinated controls, were challenged with APV. Upon challenge, the 4 × 104 and 4 × 103 TCID50 groups were protected against development of clinical signs and were resistant to reinfection. The group previously vaccinated with 4 × 102 TCID50 developed clinical signs after challenge that were considerably milder than those seen in the groups that had previously been vaccinated with lower doses or no virus. Even though 4 × 102 TCID50 vaccine dose administered by intranasal ocular route resulted in infection, incomplete protection resulted with this pivotal dose. Upon challenge, the 4 × 101 and 4 × 100 TCID50 groups exhibited milder disease signs than those seen in the challenged unvaccinated controls. In these groups, APV was detected in preparations of swabs collected at 5 days postchallenge (PC) and seroconversion was detected at 2 wk PC. These results indicate that the dose of APV vaccine that causes protection is higher than that required to produce infection.