Anticytomegalovirus experiments were carried out on 38 classes of compounds containing 320 materials of known or potential biological activity. All experiments were carried out in tubes using WI-38 cells with the test compounds added within minutes after the virus and then at additional times in medium changes 2 and 4 days later. Antiviral activity was determined by microscopic demonstration of inhibition of viral cytopathogenic effects. Thirty compounds were considered markedly active against the virus. These were the amino acid antagonists aminopterin and N-[3,5-dichloro-4-(2,4-diamino-6-pteridinyl-methylmethylamino)benzoyl]glutamic acid; the unsubstituted lactone, camptothecin; 10 purine analogs, including 8 thiopurines, 9-Β-D-arabinofuranosyladenine and purine-6-carboxaldehyde thiosemicarbazone; 13 pyrimidine analogs and 4 aldehyde thiosemicarbazones.