Six new galactoglycerolipid analogs, in which one or two 4-methylpentanoyl or trans-2-butenoyl groups are linked to the 2-O-beta-D-galactosylglycerol skeleton, were tested for their anti-tumor-promoting activity using a short-term in vitro assay for Epstein-Barr virus early antigen (EBV-EA) activation. All these compounds were more active than their linear or saturated reference compounds in inhibiting the EBV activation promoted by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), the diester 1-O-(4-methylpentanoyl)-2-O-[6-O-(4-methylpentanoyl)-beta-D-galactopyranosyl]-sn-glycerol resulting the most active glycoglycerolipid analog till now tested. Four compounds (three butenoates and one 4-methylpentanoate), when tested in the in vivo two-stage carcinogenesis test, exhibited also inhibitory effects on mouse skin tumor promotion.