[en] We calculated the distribution of the spectral density of the ultrasonic energy transmitted by the reference specimen which was water. Let σsub(i0)² be this parameter for the frequency fsub(i). In the same manner, we evaluated σsub(i)² for the ultrasonic energy transmitted by the biological tissue. We superposed on the curve Log σsub(i0)²=F(fsub(i)), each of the curves Log σsub(i)²=F(fsub(i)), corresponding to the different kinds of tissues studied. By doing so, we brought into light a ''zone'' between those curves. The judicious exploitation of that ''zone'' by calculating the spectral density coefficients β, made it possible to differentiate one tissue from another. We compared the results so obtained using a probe vibrating at 1.5 MHz with those we obtained by another probe and another method. Both sets of results are in agreement within an approximation of 10%. (author)