  In this paper we present the results of a 4-year (1996 - 1999) radio flux
density monitoring program for a sample of X- and $\gamma$-ray loud blazars.
Our program started in January 1996 and was carried out on monthly basis at the
frequencies of 5 GHz and 8.4 GHz with the 32-m antennas located in Medicina
(Bologna, Italy) and Noto (Siracusa, Italy). 22 GHz data collected in Medicina
from January 1996 to June 1997 will also be presented. The sample of selected
sources comprises most radio loud blazars with $\delta \ge -10^{\circ}$
characterised by emission in the X- and $\gamma$-ray regimes, and target
sources for the BeppoSAX X-ray mission. All sources in the sample, except
J1653+397 (MKN 501), are variable during the four years of our monitoring
program. We classified the type of variability in each source by means of a
structure function analysis. We also computed th$\alpha_{5}^{8.4}$ for all
epochs with nearly simultaneous observations ate spectral index and found that
$\alpha_{5}^{8.4}$ starts flattening at the very beginning o f a radio flare,
or flux density increase.
