Three new manumycin class antibiotics, namely manumycins E, F and G, were isolated from the culture broth of Streptomyces sp. strain WB-8376. Their structures were established by spectroscopic methods, and the S configuration of C-4 in the epoxycyclohexenone moiety was determined by CD exciton chirality method for each of the three compounds. Manumycins E, F and G are active against Gram-positive bacteria, and have moderate inhibitory effects on the farnesylation of p21 ras protein. They demonstrated weak cytotoxic activity against human colon tumor cell HCT-116.