A search for chargino, neutralino and scalar lepton pair-production in e+e− collisions at the centre-of-mass energy of 189 GeV is performed under the assumptions that R-parity is not conserved in decays and only one of the coupling constants λijk,λ′ijk or λ′′ijk is non-negligible. No signal is found in a data sample corresponding to an integrated luminosity of 176.4 pb −1 . Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived.