An approach is proposed for the calculation of an effective dielectric func
tion for a composite medium containing randomly oriented particles with uni
axial polarisability (the medium is isotropic on average). A scalar renorma
lised average polarisability of the particles is introduced, which takes co
mplete account of the tensor nature of the dipole-dipole interaction betwee
n them. Fluctuations originating from their random positions are treated in
the spirit of the average T-matrix approximation. The renormalised polaris
ability is related to the effective dielectric function via the Maxwell-Gar
nett equation. The theory is illustrated by calculating the reflectivity an
d absorption of a colloidal solution of hexagonal CdS particles.