  We present constraints on decaying-particle models in which an enhanced
relativistic density allows an $\Omega=1$ Cold Dark Matter universe to be
reconciled with acceptable values for the Hubble constant. Such models may
contain extra small-scale power, which can have important consequences for
enhanced object formation at high redshifts. Small-scale galaxy clustering and
abundances of high-redshift damped Lyman-$\alpha$ absorption clouds give a
preferred range for the mass of any such decaying particle of 2 to 30 keV and a
lifetime of 0.5 to 100 years for models with a high Hubble constant ($h>0.75$).
A lower Hubble constant, $h \simeq 0.5$, weakens the constraint to $0.5< m <
30$ keV, $0.2 < \tau < 500$ years. In permitted versions of the model,
reionization occurs at redshifts $\sim 10-200$, and this feature may be of
importance in understanding degree-scale CMB anisotropies.
