The prevalence of protein A producing strains of S. aureus was found to vary. An increase in the proportion of weak protein A producers during the past decade was found to coincide with a shift in dominant phage types in Denmark. Within phage groups I and II the ability to form large amounts of protein A was shown to be correlated to a deficient haemolysin production. This correlation was also found for group III strains susceptible to phage 75 A. Among group III strains susceptible to phages within the complex 83A, 84, 85, 6557, 89 two subgroups were remarkable. The strains which were resistant to neomycin (antibiogram PSTEN) produced large amounts of protein A, whereas those which were resistant to methicillin (antibiograms PSTEM and PSTM) produced little or no protein A. Analysis of the present material revealed no demonstrable or suspected pathogenic properties of protein A.