In staphylococcal toxic shock syndrome, hypotension and shock due to capillary leak may rapidly lead to death of the host. To investigate its pathogenesis, the cytotoxic effects of toxic shock syndrome toxin 1 (TSST-1) on porcine aortic endothelial cells (PAEC) were examined in vitro. TSST-1 killed PAEC (as measured by 51Cr release) in a dose- and time-dependent fashion and was blocked by anti-TSST-1 antibodies. Receptor-mediated endocytosis may be critical for the cytotoxic effects of TSST-1, as killing was inhibited by cold (4°C) and by addition of chloroquine and methylamine. Furthermore, calcium and oxygen appeared necessary for TSST-1 effects on PAEC. Membrane receptor binding studies indicated PAEC bind TSST-1 with high affinity (Kd = 5.7 × 10-7 M) and had 2.2 × 104 receptors/cell. Last, as measured by 125I-labeled albumin flux in a transendothelial permeability model, TSST-1 enhanced the permeability of PAEC monolayers in a dose- and time-dependent manner.