Data on methicillin-resistant Staphylococcus aureus have not been presented before in Argentina. Due to non adequate microbiologic techniques, this phenomenon may be overlooked. Heterogeneous strains are only detected with 5% NaCl agar, low temperature (30 degrees C) or prolonged incubation and/or high inocula (10(6) C.F.U.). Minimal inhibitory concentrations (MICs) of methicillin were studied by the agar dilution technique in antibiotic medium number 4 with and without 5% salt. This study was performed for 107 strains at 37 degrees C, after 24 and 48 h incubation, with an inoculum size of 10(6) CFU. S. aureus strains were considered resistant whenever MICs were greater than or equal to 8 mg/l. Forty seven (44%) methicillin-resistant strains were detected. Thirty one were high level resistant mutants and sixteen showed the heterogeneous phenomenon. Due to the high frequency of resistant strains among clinical specimens in this country, the routine use of elaborate screening tests with special media and or prolonged incubation time is justified.