  The extragalactic microlensing scenario for natural wormholes is examined. It
is shown that the main features of wormhole lensing events upon the light of
distant Active Galactic Nuclei (AGNs) are similar to some types of already
observed Gamma Ray Bursts (GRBs). Using recent satellite data on GRBs, an upper
limit to the negative mass density -- ${\cal O} (10^{-36})$ g cm$^{-3}$ --
under the form of wormhole-like objects is presented.
