Methicillin-resistant strains of Staphylococcus aureus isolated at the Royal Prince Alfred Hospital since 1965 were differentiated by phage-typing and by their lysogenic status. Most of these strains were isolated during two periods, 1965-72 and 1976-85. Nearly all of the strains isolated in the first period had one of four phage-typing patterns. Strains with each typing pattern carried two prophages; these eight phages were all different, as characterised by serological grouping and lytic spectrum. Lysogenisation of the non-lysogenic strain 1489 with each of these phages narrowed its phage-typing pattern; the typing pattern of the double lysogens was generally similar to and occasionally identical with that of the host strain that had yielded the pair of phages. In the second period, strains with one of five other phage-typing patterns predominated. Representatives of each of these carried the lysogenic phage C. The first methicillin-resistant strain carrying this phage had been isolated in 1974. The current methicillin-resistant S. aureus strains thus appear to form a distinct group that can be differentiated from those seen in earlier years.