It is known that there is a transition from photon antibunching to bun
ching in the resonance fluorescence of a driven system of two two-leve
l atoms with dipole-dipole interaction when the atomic distance decrea
ses and the other parameters are kept fixed. We give a simple explanat
ion for the underlying mechanism that in principle can also be applied
to other systems. [S1050-2947(98)01711-9].