Two hundred twenty-one healthy children, from 6 months to 7 years of age, were tested for delayed-type hypersensitivity (DTH) by the Multitest CMI (cell-mediated immunity) (Merieux Institute, Miami). This device permits the simultaneous application of seven standardized recall antigens and a glycerol diluent control. Younger children were tested on the back and older children on the volar surface of the forearm. Only 6.8% of the children were anergic, and most of them (11/15) were female. The DTH responses were present for one or more antigens in 93% of the infants. The DTH responses increased tremendously during the second year of life and increased slowly thereafter. A relatively high incidence of positive reactions was found for three of the tested antigens--diphtheria toxoid (79%), tetanus toxoid (62%), and Proteus (57%), in children in the preschool years, and accounted for three fourths of all positive reactions. Much lower levels were found for Streptococcus (25%), Candida (16%), Trichophyton (5%), and tuberculin (4%). Measurement of DTH by the standardized Multitest CMI system seems to be a convenient and reliable tool for assessing CMI function in infants and small children. The tool permitted us to measure patterns of DTH responses from infancy onward in a healthy population and to develop index values in a normal reference population with which any tested preschool child can be compared.