Acyclic neplanocin analogues were prepared by condensation of adenine or N2-acetylguanine with (E)-1,4-dichlorobut-2-ene and subsequent hydrolysis. The N-9-substituted product 9-[(E)-4-hydroxybut-2-enyl]adenine was obtained when adenine was employed as the starting purine, while N2-acetylguanine yielded both the N-7 and N-9 isomers. Cell-culture studies revealed that only the chloro-substituted intermediate 9-[(E)-4-chlorobut-2-enyl]adenine exhibited significant cytotoxicity against P-388 mouse lymphoid leukemia cells, while the N-9-substituted guanine analogue 9-[(E)-4-hydroxybut-2-enyl]guanine inhibited replication of herpes simplex viruses type 1 and type 2.