  Magnetoresistance measurements of highly underdoped superconducting
La_{2-x}Sr_xCuO_4 films with $x = 0.051$ and $x = 0.048$, performed in dc
magnetic fields up to 20 T and at temperatures down to 40 mK, reveal a
magnetic-field induced transition from weak to strong localization in the
normal state. The normal-state conductances per CuO_2--plane, measured at
different fields in a single specimen, are found to collapse to one curve with
the use of a single scaling parameter that is inversely proportional to the
localization length. The scaling parameter extrapolates to zero near zero field
and possibly at a finite field, suggesting that in the zero-field limit the
electronic states may be extended.
