Novel and known analogues of thiocolchicine were evaluated in vitro in a tubulin binding assay and in vivo in mice for acute toxicity and in the P388 lymphocytic leukemia assay. This evaluation included N-acyldeacetylthiocolchicines, N-(alkoxycarbonyl)deacetylthiocolchicines, thiodemecolcine and its methyl carbamate, and O-ethyl ethers of demethylthiocolchicines. Selective ether cleavage of thiodemecolcine with concentrated sulfuric acid at 50 degree C afforded the 2-demethyl congener, characterized as its N,O-diacetyl derivative. Several of the compounds showed high potency in the tubulin binding assay, matching the potency of colchicine. Several N-(alkoxycarbonyl)deacetylcolchicines (carbamates) exhibited strong binding affinity to tubulin but had only weak activities against the P388 tumor system, suggesting that other factors besides tubulin binding may be important for the biological effects. The compounds potent in the tubulin binding assay and in the P388 leukemia assay in mice were generally also toxic to mice in the acute toxicity test, showing thus a similar behavior of thiocolchicines to that observed earlier with colchicines. A considerable amount of data collected for 2-demethyl- and 3-demethylthiocolchicine suggests that the latter represents a broad-spectrum antitumor agent of considerable promise and possibly a less toxic substitute for colchicine.