By the addition of toxins of S. typhi, S. paratyphi A, B, and E coli to the bone-marrow tissue culture of guinea pigs the author pursued the influences of these toxins on the functions of megakaryocytes, and obtained the following results. In the case of the toxins of S. typhi and S. typhi and S. paratypbi A, the appearance of vigorous platelet separation is markedly limited, indicating a marked decrease in the megakaryocyte functions of the bone marrow loaded with these toxins. On the other hand, in the case of addition of the toxin of S. paratyphi B the decrease in the megakaryocyte function is slight, whereas the toxin of E. coli brings about no fall in the megakaryocyte function but rather accelerates it slightly. From these findings it is believed that direct inhibitory action of these toxins on the megakaryocytes constitutes one of the important factors in the mechanism of thrombocytopenia in typhoid fever.