Protein A is associated in a covalent way to the Staphylococcus aureus wall and is exposed in the bacterial surface. Its increasing use as an immunochemical reagent is due to its great affinity for immunoglobulins of multiple animal species. In this work we are presenting a method for the semiquantitative determination of bacterial surface protein A, describing the content of this protein in three certified strains and 15 new S. aureus isolations. Protein A titers were determined by a microassay as the inverse of dilutions of standardized bacterial suspensions that caused agglutination at the end point in ram erythrocytes, sensitized with anti-erythrocyte serum. Using certified strains, protein A+ (Cowan I) and protein A-(Wood 46), reproducibility of the method was assured and the relative hemagglutinating potence of new isolations was determined showing it was constant and typical of the strain. Titrations fluctuated from non detectable to a maximum of 256. Using the described method, protein A in S. aureus isolations can be quantitated to analyze its distribution and obtain new producing strains from this protein.