Circular polarization measurements were made with the Parkes 64 m telescope on 66 mainly extragalactic sources, 33 of which were studied at two or more frequencies. Circular polarization was definitely found in the eight quasi-stellar sources PKS0237-23, 0537-441, 1127-14, 1226+02 (3C273), 1253-05 (3C279), 2134+004, 2145+06 and 2345-16, as it was measured at the 4u level or higher in at least two observing sessions. For all eight sources the total power spectrum shows the effects of self~absorption. For none of the eight is there evidence of a reversal of the sense of polarization with frequency. 'For several of the sources the degree of circular polarization changed by a factor ~ 2 between observing sessions, while any accompanying change in total flux density was ;S 20 %. Instrumental effects are discussed in some detail. Fluctuations due to system noise provide the main limitation, but for strong sources uncertainties in the determination of the zero of polarization are important.

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