Comparison of the in vivo and in vitro immunosuppressive activities of the five rifamycins with their in vitro antibacterial and anti-ribonucleic acid polymerase activities indicated that correlation was poor. Examination of their activities on mitogen-induced blastogenesis in human peripheral blood leukocytes and inhibition of delayed-type hypersensitivity to partially purified protein derivative in immunized mice demonstrated that correlation was usually good. Antibacterial activity in cultures and the activities of the rifamycins inhibiting deoxyribonucleic acid-dependent ribonucleic acid polymerase appeared to correlate well. However, when these two types of activity, antiimmune and antibacterial, were compared, correlation was poor on occasion and indicated that the antiimmune activities and antibacterial activities of the rifamycins are probably not related
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