A series of ten S-substituted derivatives of the alpha and beta anomers (1a and 1b) of 2'-deoxy-6-thioguanosine has been prepared by S-alkylation of the parent nucleosides and/or by mercaptide displacement reactions on 6-chloro intermediates. Against L1210 murine leukemia all beta anomers were active but potency was reduced relative to 1b. Most S-alkyl alpha anomers were inactive in this test. Limited testing against P388 murine leukemia showed all alpha-anomer derivatives to be inactive but the beta anomers were more effective than the parent. S-Substitution sharply reduced acute toxicity in both series. In vitro DNA and RNA synthesis inhibition data are also reported. The antitumor activity of these derivatives and of the 2',5'-di-O-acetyl derivatives of 1a and 1b against lymphoid leukemia L1210 is reported. Some results with the lymphocytic leukemia P388 and an in vitro assay of the inhibition of nucleic acid synthesis are also given.