Skin testing and in vitro lymphocyte blastogenesis with Monilia and tetanus toxoid were examined as methods of screening cell mediated immunity (CMI) in infants and children. With increasing age, all assays were more likely to be positive. Tetanus toxoid was a better single screening antigen than Monilia, particularly for young infants. Combining all assays, at least one was positive in 88% of children over 6 weeks of age. Combining the two skin tests, at least one was positive in 73% of children in the same age range. Results for the blastogenic assays indicated that this in vitro test is more sensitive than the in vivo skin test for demonstrating CMI to these two antigens. Infants with documented histories for Monilia diaper rash had a significantly higher probability of having a positive skin or blastogenic reaction with Monilia.