[en] The NEMO collaboration plans to build an experimental set-up managing 10 kg of 99% enriched ¹⁰⁰Mo to study ββ(Ον) decay, in order to probe effective neutrino mass of the order of 0.1 eV. To reach this goal, improvements were made resulting in the NEMO-II setup which has been running successfully since August 1991. After 2000 hours no 2e-event has been observed with an energy greater than 2.3 MeV on a very pure copper foil. Simultaneously, the level of pollution of a natural molybdenum foil not specially purified was measured, and the results are in good agreement with a germanium γ-spectroscopy measurement. Delayed emission of α particles was also observed using eγα-events. (R.P.) 3 refs.; 4 figs