Skin test reactivity to three mycobacterial sensitins (M. avium, M. fortuitum and M. scrofulaceum) was studied in 353 healthy children vaccinated with the BCG vaccine at birth. A significant waning of reactivity to all of the three sensitins was found to occur with increasing age. Revaccination against measles, parotitis and rubella had been given to 31 (9%) of the children, all aged > 5.5 years. They had significantly larger reactions sizes, which was contrary to what was expected. Children with atopic dermatitis (n = 19) had significantly smaller reaction sizes to the M. fortuitum sensitin. Neither respiratory viral infections during the preceding six months nor contact with pets or farm animals had any influence on reaction sizes. The variation of sensitin reactions was large in preschool children vaccinated with the BCG vaccine at birth. The diagnostic value of sensitin reactions in preschool children needs to be elaborated in clinical studies in children with clinically demonstrable mycobacteriosis.