Plasmid analysis, bacteriophage typing and antibiograms were used to detect the source and rate of Staphylococcus aureus colonization and disease in newborn infants in a nursery. Staphylococcal colonization on the nasal mucosa and umbilicus of 140 neonates took place within 12 hours after birth. The colonization rates reached 100% from the fourth day onward and fell to 66.7%, 25.5% and 17.5% by the age of 1 month, 6 months and 1 year, respectively. The incidence of nursery-acquired infection caused by S. aureus was at least 16.4%. All infections were confined to the skin or mucosa and were not serious. It is believed that the S. aureus which colonized and infected the newborn infants mainly originated from the nares and the hands of nurses, because only S. aureus strains from nurses and infected infants had the same phage patterns (52/52A/47/53/75/77/81) and carried 2 plasmids of the same size (1.4 and 1.7 Md), whereas S. aureus isolated from other sources did not have the same phage patterns or plasmid profiles. In view of the results of this study, it is considered that the importance of careful washing of the hands of personnel before and after handling infants should be emphasized.