Possible LSND effect as a small perturbation of the bimaximal texture for three active neutrinos

A particular form of mixing matrix for three active and one sterile neutrin os is proposed. Its 3 x 3 part describing three active neutrinos arises fro m the popular bimaximal mixing matrix that works satisfactorily in solar an d atmospheric experiments if the LSND effect is ignored. Then, the sterile neutrino, effective in the fourth row and fourth column of the proposed mix ing matrix, is responsible for the possible LSND effect by inducing one ext ra neutrino mass state to exist actively. The LSND effect, if it exists, tu rns out to reveal its perturbative nature related to small mixing of three active neutrinos with their sterile partner.