At the Department of Clinical Bacteriology of Medical University of Warsaw, S. aureus strains displaying negative reactions in the coagulase tube-test were isolated with a relatively high frequency from clinical specimens. As many as seventeen of such strains all from different patients were isolated during 1994 among MRSA strains. Despite coagulase negativity all strains were positive in tests for clumping factor (CF), protein A and thermonuclease. Biochemical characteristics, phage patterns and antibiotic resistance characteristics of these strains were tested. Most of the coagulase-negative strains possessed a heterogenous type of methicillin resistance. Apart from methicillin most of them were resistant to many other antimicrobials. All were resistant to gentamicin, tetracyclines and macrolide-lincosamide-streptogramine B. An occurrence of coagulase-negative S. aureus strains may lead to problems in their identification and the necessity of an application of other methods like CF, protein A or biochemical reactions.