As shown recently (W. Kummer, H. Liebl, D,V. Vassilevich, Nucl, Phys, B 544
(1999) 403) 2d quantum gravity theories - including spherically reduced Ei
nstein-gravity - after an exact path integral of its geometric part can be
treated perturbatively in the loops of (scalar) matter. Obviously the class
ical mechanism of black hole formation should be contained in the tree appr
oximation of the theory. This is shown to be the case for the scattering of
two scalars through an intermediate state which by its effective black hol
e mass is identified as a "virtual black hole". The present discussion is r
estricted to minimally coupled scalers without and with mass. in the first
case the probability amplitude diverges, except the black hole is "plugged"
by a suitable boundary condition. For massive scalars a finite S-matrix el
ement is obtained. (C) 2000 Elsevier Science B.V. All rights reserved.