3"-O-Acylation of leucomycin A5 increased its in vitro antibacterial activity against sensitive microorganisms and also some resistant ones. An increased effect was particularly noted when an acetyl or propionyl group was introduced. On the contrary, acylation of the C-3 and C-9 hydroxyl groups reduced the antibacterial activity in vitro. Introduction of an acyl group at the C-3" hydroxyl group increased the serum level of the compound. The increase in serum level from 3"-O-acylation is higher than that from 3-O-acylation. The serum level of the 3"-O-propionyl derivative of leucomycin A5 was higher than that of the 3"-O-acetyl derivative. 3"-O-Propionylleucomycin A5 (5) was the derivative that showed the highest antibacterial activity and yielded the highest serum level among the derivatives examined.