A series of 5-substituted 2-amino-4,6-dihydroxypyrimidines were prepared by a modified condensation of the corresponding monosubstituted malonic acid diesters with guanidine in an excess of sodium ethoxide. The optimized procedure using Vilsmeier–Haack–Arnold reagent, followed by immediate deprotection of the (dimethylamino)methylene protecting groups, has been developed to convert the 2-amino-4,6-dihydroxypyrimidine analogs to novel 5-substituted 2-amino-4,6-dichloropyrimidines in high yields. Pilot screening for biological properties of the prepared compounds was done in mouse peritoneal cells using the in vitro nitric oxide (NO) assay. Irrespective of the substituent at the 5 position, 2-amino-4,6-dichloropyrimidines inhibited immune-activated NO production. The most effective was 5-fluoro-2-amino-4,6-dichloropyrimidine with an IC₅₀ of 2 µM (higher activity than the most potent reference compound) while the IC₅₀s of other derivatives were within the range of 9–36 µM. The 2-amino-4,6-dihydroxypyrimidine counterparts were devoid of any NO-inhibitory activity. The compounds had no suppressive effects on the viability of cells. The Mechanism of action remains to be elucidated.