Independently obtained mutations (apt) of resistance to DAP (2,6-diaminopurine) and MP (6-methylpurine), that affect adenine phosphoribosyltransferase (APRT) in Escherichia coli, are different in their effect on the conversion of several substrates of APRT, such as DAP, MP, MAP (6-methylaminopurine) and adenine, to their nucleotide derivatives. Most of mutants were resistant to DAP and MP, unable to utilize MAP (as purine source) and differed in their ability to uptake adenine from the medium. Among the mutants capable to utilize adenine the following types are found: (1) resistant to DAP and MP, but capable of utilizing MAP, and (2) resistant to DAP, capable of utilizing MAP, but sensitive to MP. The gene apt encoding APRT is located between genes proC and purE; the frequency of cotransduction between proC and several apt mutations is found to be 1.7--2% and purE-apt--to be 5--10.8%. Mutations apt block up the ability of purine-dependent (pur) bacteria lacking purine nucleoside phosphorylase (pup) to use purine ribonucleosides as purine sources. The degree of that blocking depends on the ability of apt mutants to convert adenine to AMP via APRT. These observations confirm our previous data, that the ability of pur pup mutants to use purine ribonucleosides depends on the activity of APRT.