  We make a detailed calculation of the amplitude of isocurvature perturbations
arising from inflationary models in which the cold dark matter is represented
by a scalar field which acquires perturbations during inflation. We use this to
compute the normalization to large-angle microwave background anisotropies.
Unlike the case of adiabatic perturbations, the normalization to COBE fixes the
spectral index of the perturbations; if adiabatic perturbations are negligible
then $n_{iso} \simeq 0.4$. Such blue spectra are also favoured by other
observational data. Although the pure isocurvature models are unlikely to
adequately fit the entire observational data set, these results also have
implications for models with mixed adiabatic and isocurvature perturbations.
