The susceptibility of inbred lines of chickens to graded levels of infection with a virulent isolate (wild-strain) of infectious laryngotracheitis virus (ILTV) was investigated. An association was found between the level of resistance to ILTV and the line of inbred chickens, with chickens from the J1 inbred line being more resistant to clinical ILTV than M1 and N1 inbred chickens. The N1 chickens, however, became susceptible with higher doses of ILTV, while J1 chickens remained relatively resistant to clinical disease. Studies using other inbred lines of chicken which had the same major histocompatibility complex (MHC) antigens showed similar levels of resistance or susceptibility, suggesting a possible association of the chicken MHC with resistance or susceptibility to ILTV.