Discrete source contributions to small-scale anisotropies of the microwave background

We have computed the temperature fluctuations of the microwave background due to unresolved, randomly distributed sources, for a wide interval of angular scales (from a few arcseconds to several tens of degrees) and for wavelengths ranging from several centimeters to below 1 mm. For λ >= 6 cm and {PHI}_0_ >= 30" the present estimates follow almost directly from the observed radio source counts. By properly taking into account all of the available data, we have been able to narrow down substantially the uncertainties in the extrapolations to smaller scales and higher frequencies. We find that only within a relatively narrow frequency range (from ~ 20 to ~ 200 GHz) the contribution of discrete sources to the temperature fluctuations on scales of less than 1^deg^ is expected to be {DELTA}T/T <= 10^-5^. We call attention to the effect of clustering that might strongly enhance fluctuations at least on some angular scales. On the other hand, we show that the most sensitive data on {DELTA}T/T at λ = 6 cm provide significant constraints on the correlation function of radio sources.