[en] The GALLEX experiment, installed in the Gran Sasso Underground Laboratory, has measured the rate of production of ⁷¹Ge from ⁷¹Ga by solar neutrinos, using 30.3 tons of gallium in the form of gallium chloride solution. 14 measurements carried out in one year of operation give, after corrections for side reactions and other backgrounds, an average value of (83±19(stat.)±8(syst.)) SNU (1σ) due to solar neutrinos. This result, two standard deviations below the predictions of solar model calculations (122--132 SNU), constitutes the first observation of solar pp-neutrinos. The possible deficit is interpreted, together with those of the chlorine and Kamiokande experiments, in terms either of astrophysics or of neutrino oscillations (MSW effect)