369 staphylococcal strains isolated from clinical material were examined for tubeagglutination with sensitized sheep red cells in a standardized assay to study its reliability for routine identification of S. aureus. Colonies isolated from blood agar plates correlated in 99.5% with the (optimized) coagulase reaction. The test is easily to perform and results can be read after 2 hours, whereas the reference methods coagulase, hyaluronidase and deoxyribonuclease took as much as 24 h. The reliability of these tests is discussed.