A series of unsaturated analogues of nucleosides were prepared and their cytotoxic, antitumor, and antiviral activities were investigated. Alkylation of cytosine with (E)-1,4-dichloro-2-butene gave chloro derivative 2f, which was hydrolyzed to alcohol 2h. Cytosine, adenine, 2-amino-6-chloropurine, thymine, and (Z)-1,4-chloro-2-butene gave compounds 4c-f, which, after hydrolysis, afforded alcohols 4a, 4b, 4g, and 4h. Alkenes 4d and 4e were cyclized to heterocycles 12 and 13. Alkylation of 2,6-diaminopurine with 1,4-dichloro-2-butyne led to chloro derivative 6a, which was hydrolyzed to alcohol 6b. Allenic isomerization of 6b gave compound 5c. Chloro derivatives 2e-g, 4c-f, 5d, and 6c-e as well as pyrimidine oxacyclopentenes 9c and 9d are slow-acting inhibitors of murine leukemia L1210 of IC50 10-100 microM. The most active were analogues 4c, 4d, 4e, and 6e (IC50 10-20 microM). The corresponding hydroxy derivatives were less active of inactive. Inhibition of macromolecular synthesis with compounds 4c, 4d, 6e, 9c, and 9d follows the order: DNA greater than RNA greater than or equal to protein. Cytotoxic effects of 4c, 6e, and 9d are not reversed with any of the four basic ribonucleosides or 2'-deoxyribonucleosides. Inhibitory activity of cytosine derivative 9c is reversed with uridine and 2'-deoxyuridine but not with the corresponding cytosine nucleosides. Zone assays in several tumor cell lines show that active compounds are cytotoxic agents with little selectivity for tumor cells. Analogue 6c showed 16.7% ILS in leukemia P388/o implanted ip in mice at 510 and 1020 mg/kg, respectively. Cytallene (5b) and 6'beta-hydroxyaristeromycin (10) exhibited significant activity against Friend and Rauscher murine leukemia viruses. The rest of the hydroxy derivatives, with the exception of 4a, were moderately effective or inactive as antiviral agents. None of the chloro derivatives or oxacyclopentenes exhibited an antiviral effect at noncytotoxic concentrations. Z-Olefin 4b and 2-aminoadenallene (5c) are substrates for adenosine deaminase.