Excess production of toxic shock syndrome toxin 1 by appropriate strains of Staphylococcus aureus occurs when the organisms are grown in an environment deficient in Mg++. Since many of the fibers previously used in tampons combine with Mg++, an explanation for the pathogenesis of menstrually related toxic shock syndrome presents itself. Pitfalls in the reproduction of these experiments have been investigated and include attention to inoculum size, to the effect of washing the inoculum to rid it of magnesium from the parent culture, and related variables. The growth of staphylococci in magnesium-deficient medium is slower than in the usual culture media, making it necessary that the effect of magnesium deficiency be examined after sufficient incubation to permit maximal toxin production to occur. When these variables are taken into account, a coherent theory to explain the pathogenesis of menstrual TSS emerges. Absorbency of tampons, which correlates with the capacity to absorb Mg++, becomes, as has been suggested, a surrogate for the magnesium effect. The observations raise the possibility of a safer tampon, one in which the magnesium content is regulated to cause the staphylococci not to be in a magnesium-deficient environment.