Syntheses and biological activities of 12 N6-(n-alkylureido)purine ribonucleosides (alkyl chain length of 1--10, 16, and 18 carbons) and three N6-(n-alkylureido)purine ribonucleoside 5'-phosphates (chain length of 4, 9, and 10 carbons) are described. The N6-(n-alkylureido)purine ribonucleosides were prepared by a reaction of (2',3',5'-tri-O-acetyl-beta-D-ribofuranosyl)-9H-purine-6-carbamate and n-alkylamine in refluxing pyridine. The 5'-nucleotides were prepared by direct phosphorylation of the corresponding ribonucleoside with phosphorus oxychloride and triethyl phosphate. Some N6-(n-alkylureido)purine ribonucleosides (n-octyl, n-nonyl, and n-decyl) and their nucleotides showed a marked antiproliferative activity against L-1210 cells in culture.