The aim of this study was to investigate the presence of S. sobrinus and S. mutans in specimens of dental plaque and saliva of children five years of age in Reykjavik, Iceland (study 1) and in samples of dental plaque from children nine years of age in Amsterdam, The Netherlands (study 2). The immuneblotting technique (IBT) was a suitable method to evaluate the presence and numbers of S. mutans and S. sobrinus in human dental plaque and saliva. In study 1, eighty-four children were evaluated bacteriologically; of these, 73 percent harbored mutans streptococci in their plaque or saliva. S. sobrinus similarly was present in 29 percent of the children. In study 2 (seventy-two children), the corresponding percentages were 81 percent for S. mutans, and 35 percent for S. sobrinus. The latter was detected in 6 percent of the plaque samples exclusive of S. mutans.