The Martin boundary theory allows one to describe all positive harmonic
functions in an arbitrary domain $E$ of a Euclidean space starting from
the functions $k^y(x)=\sfrac{g(x,y)}{g(a,y)}$, where $g(x,y)$ is the Green
function of the Laplacian and $a$ is a fixed point of $E$. In two
previous papers a similar theory was developed for a class of positive
functions on a space of measures. These functions are associated with a
superdiffusion $X$ and we call them $X$-harmonic. Denote by $\M_c(E)$ the
set of all finite measures $\mu$ supported by compact subsets of $E$.
$X$-harmonic functions are functions on $\M_c(E)$ characterized by a
mean value property formulated in terms of exit measures of a
superdiffusion. Instead of the ratio $\sfrac{g(x,y)}{g(a,y)}$ we use a
Radon-Nikodym derivative of the probability distribution of an exit
measure of $X$ with respect to the probability distribution of another
such measure. The goal of the present note is to find an expression for
this derivative.