  In a recent paper by Rodrigues-Williams \& Hogan (RH94), a correlation
between high-redshift, optically-selected QSOs and Zwicky clusters was reported
at a very high significance level. Due to the fairly bright flux threshold of
the cluster sample, these correlations cannot be interpreted as being due to an
environmental effect of the clusters on the quasar activity. The most likely
interpretation employed in RH94 was the effect of gravitational lensing by the
foreground clusters, though the required magnification to explain the observed
correlations has to be considerably higher than obtained from simple mass
models for the clusters. We have repeated the analysis of RH94 using a
different QSO sample, namely radio quasars and radio galaxies from the 1-Jy
sample. In accordance with RH, we detect a statistically significant
correlation between Zwicky clusters and 1-Jy sources with intermediate redshift
($z\approx 1$), but fail to detect significant effects for higher-redshift
sources. In addition, we detect a highly significant underdensity of
low-redshift radio sources around Zwicky clusters, for which an environmental
interpretation seems to be most reasonable. Our result for the overdensity of
$z\sim 1$ sources is in good agreement with previous results and can possibly
be interpreted as a lensing effect, though we have not tried to quantitatively
investigate a lensing model. Other interpretations, such as patchy dust
obscuration, can not explain the observed effect and its tendency with
redshift.
