The difference in optimal conditions for DNA polymerization catalyzed by AMV reverse transcriptase on poly(A) and poly(dA) templates with d(pT)10 and (pU)10 primers has been found. A comparison of the initiating abilities of d(pT)10 and (pU)10 primers under optimal conditions for various template.primer complexes has been made. The best template.primer complex was poly(A).d(pT)10 and the worst was poly(A).(pU)10. The lengthening of d(pT)n primers by a mononucleotide unit (n = 2-10) increases their affinity by a factor of about 2 and 3 in the case of poly(dA) and poly(A) templates, respectively. The affinities of d(pT) to the enzyme does not change with the primer length.