Strains showing a negative reaction in tube test for coagulase constitute 10 to 20% of all Staphylococcus aureus isolated from patients of Hospital of Infant Jesus in Warsaw. Most of them are MRSA. In 42 MRSA strains showing negative reaction for coagulase, the presence of coagulase (coa) and nuclease (nucA) genes was checked. Determination of whole cell DNA with PCR reaction was performed. The obtained results revealed that all 42 strains possessed gene nucA, but only 39 strains possessed the coa gene.