Superantigencity of staphylococcal endocarditis isolates. A thymidine-based proliferation assay using splenocytes from AE° and HLA-DR3 transgenic mice was used to assess the biological activity of superantigens in bacterial culture supernatants. The following planktonic culture supernatants were diluted in RPMI (1 in 256 dilution) and tested: IDRL-4297, IDRL-4319 and IDRL-4578 (positive for sea and tst); IDRL-4420, IDRL-4587 and IDRL-5494 (positive for seb); IDRL-4320 (positive for sec and tst) and IDRL-6982 (positive for sed). The remainder of the supernatants were from isolates that tested negative for sea, seb, sec, sed, see, seh and tst by PCR. Planktonic culture supernatant of S. aureus RN6734 containing intact cloned seb pRN5543::seb (pRN7114) was used as a positive control (PC). Supernatant from S. aureus RN6734 containing seb with a large 3′ deletion, pRN5543::seb(b.2) (pRN7116) was used as a negative control (NC). TSB indicates splenocytes cultured with trypticase soy broth alone. Each bar represents the mean±standard error of triplicate wells. CPM, counts per minute. *Not done.