Strains of Escherichia coli K-12 containing various combinations of pur (de novo synthesis of purines), pup (purine nucleoside phosphorylase), add (adenosine deaminase) and apt (adenine phosphoribosyl transferase) mutations have been constructed. The apt mutation blocks the ability of strains of pur add and pur add pup genotype to utilize both adenine and adenosine as sole purine sources. Exogenously supplied histidine (that blocks conversion of AMP to guanine nucleotides) does not reduce the growth rate of the strain of pur apt genotype on adenosine as the sole purine source. Adenine released into the cultural medium of bacteria containing simultaneously apt and pup mutations. This data suggest that cultures of E. coli are unable to phosphorylate adenosine to AMP and that they are capable to degrade adenosine to free adenine without participation of purine nucleoside phosphorylase (gene pup).