The number of S. mutans (S.m.) in saliva of parents and their children was studied. Thirty-six children, 4 1/2-5 years old, and their parents participated. A quantitative correlation was found between the number of S.m. in the mothers and their children. The degree of infection also seemed to reflect the caries experience of the child. When adults with different levels of S.m. in saliva contaminated metal spoons with saliva, a correlation between the salivary count of S.m. and the number of microorganisms transferred to the spoon was found. The survival of S.m. on saliva-contaminated test-plates was tested. After 7 h the number of viable S.m. had decreased considerably but a few cells could be found after 24-48 h. To study the presence of S.m. on objects in the city often touched by hands, MSB-containing agar dishes were pressed against several objects such as doorhandles. Only four out of 24 objects were found to be contaminated by S.m.