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Experimental septicemia was induced in mice by intraperitoneal injection of 10 to 100 lethal doses of Staphylococcus aureus and Streptococcus pneumoniae. Animals were treated by a mixture of adipic acid and spiramycin (subcutaneous route) or by spiramycin base (oral route), 1 and 6 hours after infection. To determine the effective dose 50% that achieves survival of half the mice after 7 days, each drug was used in 6 dosages (mg/kg) and each dosage was given to 12 mice. In 21 independent experiments, ED50S of spiramycin adipate by the subcutaneous route were found to be 5 to 50 times lower than those of spiramycin base per os. These results are consistent with the high serum peak concentrations of spiramycin adipate observed following subcutaneous administration.