The data presented in this study indicate that the Staphylococcus aureus is related casually to the toxic shock syndrome and that the dominant phage type was 29. This type has occurred singly as well as in combination with phage type 52 and some other group I phage types. The 29, 52 complex was present in approximately two-thirds of a series of 159 isolates of Staphylococcus aureus obtained from patients with toxic shock syndrome, while another fourth was nontypable. In further support of this concept was the evidence that 94.1 per cent of the isolates of the 29, 52 phage complex tested were capable of producing pyrogenic exotoxin C. These results gave confirmatory evidence of the etiologic importance of this phage type complex of Staphylococcus aureus. The high percentage of 25.8 per cent nontypable isolated has remained unexplained, and this is suggestive of the possibility that another important phage type may be present but obscure, since 83 per cent were shown to be capable of producing pyrogenic C toxin. The data reported also indicated that the current activity of this phage type is another manifestation of the versatility of Staphylococcus aureus and its ability to produce different pathogenic strains capable of causing different or new infections in clinical practice.