A series of 133 isolates of methicillin-resistant Staphylococcus aureus was fingerprinted by the immunoblot technique. Extracts were prepared by lysostaphin degradation of overnight cultures and peptides were separated by SDS polyacrylamide gel electrophoresis. The peptides were transblotted on to nitrocellulose membranes and probed with (1) a hyperimmune rabbit serum raised against a methicillin-resistant S. aureus isolate, (2) a hyperimmune rabbit serum raised against an isolate of S. epidermidis, and (3) serum from a patient who had recovered from an infection with a methicillin-resistant S. aureus. This typing method confirmed the existence of an epidemic strain that accounted for 102 of the isolates. The remaining 31 isolates were grouped into a further seven types which correlated with the results of phage typing and antibiograms.