The onset of protective immunity with MS-H was determined through experimental challenge and compared with the parent strain 86079/7NS. MS-H vaccinates and 86079/7NS inoculates were challenged at 1, 2, 3, 4, 5, and 6 wk after vaccination, then examined 2 wk after challenge for signs of respiratory disease. Serologic results indicated that 100% of MS-H vaccinates had antibodies to MS by 3 wk after vaccination and 100% of 86079/7NS inoculates were positive by 2 wk after inoculation. From 3 wk after vaccination, MS-H vaccinates had a significantly lower incidence of air sac lesions and, from 4 wk after vaccination, a significantly lower air sac lesion severity. In 86079/7NS-inoculated birds, a significantly lower incidence of air sac lesions was observed from 1 wk after inoculation, and air sac lesion severity was significantly lower than the unvaccinated controls at 3 wk after inoculation. It would appear that, under the conditions of this experiment, protective immunity elicited by MS-H appeared at 4 wk after vaccination, slightly later than the appearance of serum antibody. Although the MS-H vaccine was slower to establish protective immunity than 86079/7NS, there was no significant difference between the two strains by 4 wk after vaccination or inoculation.