Coagulase-negative pathogenic staphylococci were studied physiologically and serologically to determine their relationship to Staphylococcus aureus. When 46 characters were studied and tallied, the 21 coagulase-negative pathogenic strains made up a heterogeneous intermediate group sharing not all the characters of S. aureus but appreciably more than S. epidermidis. Some characters among the coagulase-negative pathogens indicating a relationship to S. aureus were serotyping, lysostaphin sensitivity, growth rates, and endogenous respiration. Seventy-one percent of the coagulase-negative pathogenic strains were resistant to penicillin; of these, 80% were multiple antibiotic resistant.From these results it would appear that the genus Staphylococcus cannot be divided satisfactorily into demarcated species but that strains within this genus form a continuous spectrum between the two presently recognizable extremes with new subtypes evolving as characters are lost. If a species must be designated under the existing system of classification, a strain producing coagulase can be called S. aureus. However, coagulase-negative pathogenic strains cannot be classified with accuracy as S. epidermidis on the basis of the absence of this single character, coagulase.

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