1. Two-way selection for delayed-type hypersensitivity wattle reaction (DWR) competence to BCG antigen was carried out over 4 generations, using a flock of White Leghorn chickens. DWR was measured by intradermal injection of BCG into the wattle of chickens. 2. Selection for DWR was effective. The generation means of average selection differential, selection response and realized heritability were 1.15 mm, 0.77 mm and 0.70 respectively. Correlated responses were found in body weight, egg weight and hatchability. In all of these traits, the L line (selected for low wattle reaction) showed higher values than the H line (high reaction). 3. Specific trends in gene frequencies were observed at the major histocompatibility B and alkaline phosphatase Akp loci. 4. Significant line differences were found in graft-versus-host reaction competence and Marek's disease resistance. In both characters, the L line was higher than the H line. No line differences were found in phytohaemagglutinin reponse, phagocytic activity or immune responses.
