  We have investigated the IRAS colours and the far-IR to optical luminosity
ratios of a complete sample of elliptical's and S0's brighter than $B_T = 12$.
On the average, elliptical galaxies emit in the far-IR less than 1\% of their
bolometric luminosity, while S0's are about a factor of three brighter in the
far-IR. On the average, the photospheric emission of red giant stars can
account for 50--60\% of the $12\mic$ flux from early type galaxies; the
contribution from diffuse dust at this wavelength is $< 10\%$ in the case of
ellipticals,and may amount to 20--40\% for S0's. An additional, $\sim
30--40\%$, contribution from circumstellar emission from evolved giants with
mass loss (particularly OH/IR stars) seems to be required in the case of
ellipticals. This suggests a small but significant star formation activity in
these galaxies at a look-back time of 1--$2\,$Gyr, corresponding to about 10\%
of that typical of a disk galaxy having the same V-band luminosity. As for
S0's, the larger diffuse dust emission may swamp to some extent that of
circumstellar dust, which is indicated to be, on the average, $\lsim 20\%$. The
weak emission from diffuse interstellar dust, detected mostly at $60\mic$ and
$100\mic$, has color temperatures similar to those of disk galaxies; as in that
case of the latter, a warm dust component is suggested, associated to
star-forming regions. The implied star formation would be a few percent of that
of disk galaxies of similar V-band luminosity and could account for a fraction
of the observed UV branch of early type galaxies.
