We study the large λ limit of the loop-dependent characteristic functional Z(λ) =< exp(iλ c v · d x) >, related to the probability density function (PDF) of the circulation around a closed contour c. The analysis is carried out in the framework of the Martin-Siggia-Rose field theory formulation of the turbulence problem, by means of the saddle-point technique. Axisymmetric instantons, labelled by the component σzz of the strain field -a partially annealed variable in our formalism -are obtained for a circular loop in the xy plane, with radius defined in the inertial range. Fluctuations of the velocity field around the saddle-point solutions are relevant, leading to the lorentzian asymptotic behavior Z(λ) ∼ 1/λ 2 . The O(1/λ 4 ) subleading correction and the asymmetry between right and left PDF tails due to parity breaking mechanisms are also investigated.