  A particular form of mixing matrix for three active and one sterile neutrinos
is proposed. Its $3\times 3 $ part describing three active neutrinos arises
from the popular bimaximal mixing matrix that works satisfactorily in solar and
atmospheric experiments if the LSND effect is ignored. Then, the sterile
neutrino, effective in the fourth row and fourth column of the proposed mixing
matrix, is responsible for the possible LSND effect by inducing one extra
neutrino mass state to exist actively. The LSND effect, if it exists, turns out
to reveal its {\it perturbative} nature related to small mixing of three active
neutrinos with their sterile partner.
