This study examined cell-mediated immune responses of aged guinea pigs following intranasal sensitization with Bacillus Calmette Guérin (BCG). Young adult and aged guinea pigs, one and three years old, respectively, were inoculated intranasally with BCG. Changes in lung cell profile, production of migration inhibition factor (MIF) by lung derived lymphocytes and development of delayed hypersensitivity-skin (DHS) reaction to purified protein derivative (PPD) were evaluated at various time intervals. Normal lung lavage cell profiles were similar in both groups. Significant increases in total lung lavage cells occurred in both groups at 2 and 6 weeks following sensitization and corresponded with significant increases in the number of macrophages. The young adult group had significant increases in the total number of lymphocytes and rosette forming cells at 6 weeks compared to their preimmunization levels. Production of MIF was significantly greater in magnitude in the young adult group at 2 weeks compared to aged groups. The total number of animals mounting immune responses to BCG (MIF production) was also significantly lower in the aged group over the 6 week study period. DHS reaction to PPD was positive in all young adult animals, while only half of the aged guinea pigs were positive at 6 weeks. Data suggest that age adversely affects lung resistance to infection from intracellular microbial agents of the respiratory tract.