The antitumor effect of a synthetic cord factor (6, 6'-Di-O-decanoyl-alpha, alpha-trehalose) (SS 554) on the growth of Meth-A fibrosarcoma in BALB/c mice was examined. With regard to administration routes, only intratumoral (i.t.) injection showed a curative effect; subcutaneous (s.c.), per oral (p.o.) or intravenous (i.v.) routes has no such effect. To show the antitumor effect of known natural and synthetic cord factors, the co-presence of oily vehicles has been shown to be necessary. Accordingly, compound SS 554 examined in suspensions of sesame oil, squalane (SQA), squalene (SQE) or sesame oil and water emulsion had a curative effect with a 60% survival rate. However, no such effect was obtained with a suspension in PBS or in HCO-60 solution. In this regard, it should be noted that sequential but independent administration of SS 554 and oil was found to be equally as effective as simultaneous administration of oil with SS 554. Thus the effect of the oil should be reconsidered through an examination of the sequential appearance of effector cells. In the case of sesame oil, the amount of oil necessary was over 10%, or 0.01 mg absolutely. When the dose effect of SS 554 was examined in the presence of 10% sesame oil, doses over 1 mg exhibited a dose dependent curative effect. In tumor-bearing mice, the effect of the time of administration was also examined; the best result was obtained when intratumor injection was performed on day 3 after tumor implantation. Mice that recovered after SS 554 treatment exhibited growth inhibition and rejection of rechallenged Meth-A cells. However, this immunity was specific as it did not extend to a rechallenge with RL male-1 leukemia cells.