  A superconducting transition in the temperature dependence of the ab-plane
resistivity of underdoped YBa_2Cu_3O_{6+x} crystals in the range T_c<30 K has
been investigated. Unlike the case of samples with the optimal level of doping,
the transition width increased insignificantly with magnetic field, and in the
range T_c<13 K it decreased with increasing magnetic field. The transition
point T_c(B) was determined by analyzing the fluctuation conductivity. The
curves of B_{c2}(T) measured in the region T/T_c>0.1 did not show a tendency to
saturation and had a positive second derivative everywhere, including the
immediate neighborhood of T_c. The only difference among the curves of
B_{c2}(T) for different crystal states is the scales of T and B, so they can be
described in terms of a universal function, which fairly closely follows
Alexandrov's model of boson superconductivity.
