In this paper, we review our earlier work wherein we examined the sulfides in the acetone extract of garlic (A. sativum L.), and obtained new major sulfoxides of 3,4-dimethylthiolane-type (representative: garlicnin B₁), together with new sulfides of acyclic-type, 2-methylthiolane (and thiane)-type, 1,2-dithiolane-type, 2-oxothiolane-type, and kujounin A₁ derivative, and known (E)-ajoene. The isomer of garlicnin B₁, onionin A₁, obtained from onion showed the potential in inhibiting the polarization of M2-activated macrophages that was capable of suppressing tumor-cell proliferation. In that study, the effects of onionin A₁ on tumor progression and metastasis in both osteosarcoma and ovarian cancer-bearing mouse models were also examined. Tumor proliferation was depressed, and tumor metastasis was controlled by regulating macrophage activation. These results showed that onionin A₁ was an effective agent for controlling tumors in both in vitro and in vivo models, and that the antitumor effects observed in vivo were likely caused by reversing the antitumor immune system. Activation of the antitumor immune system by onionin A₁ might be an effective adjuvant therapy for patients with osteosarcoma, ovarian cancer and other malignant tumors.