The action of adenosine deaminase on racemic carbocyclic analogues of 6-aminopurine nucleosides was investigated. When either racemic carbocyclic adenosine [(+/-)-C-Ado] or the racemic carbocyclic analogue [(+/-)-C-2,6-DAP-2'-dR] of 2,6-diaminopurine 2'-deoxyribofuranoside was incubated with this enzyme, approximately half of the material was deaminated rapidly. From the resulting solution, the D isomers of the deaminated carbocyclic analogues (D-carbocyclic inosine, D-C-Ino, or D-carbocyclic 2'-deoxyguanosine, D-2'-CDG) and the L isomers of the undeaminated carbocyclic analogues were isolated. At higher concentrations of the enzyme, deamination of L-C-Ado and L-C-2,6-DAP-2'-dR proceeded slowly, thus also making the other enantiomers accessible. In tests in vitro against herpes simplex virus, types 1 and 2, D-2'-CDG was as active and potent as (+/-)-2'-CDG, whereas L-2'-CDG displayed only modest activity. In contrast to the previously reported high activity and potency of (+/-)-C-2,6-DAP-2'-dR against these two viruses, L-C-2,6-DAP-2'-dR was inactive.