We report a detailed optical study of the clusters Abell 2125 and 2645 .
These clusters are very similar in redshift ( z \approx 0.25 ) and richness ( Abell class 4 ) , yet contrast strongly in blue fraction and radio-galaxy populations . [ Butcher & Oemler ( 1984 ) ] report that A2125 and A2645 have blue-galaxy fractions of 0.19 and 0.03 respectively , while radio observations with the VLA and subsequent optical identifications on the digital Palomar Sky Survey show an apparent excess of radio galaxies in A2125 relative to A2645 ( [ Dwarakanath & Owen 1996 ] ) .

Our spectroscopic observations confirm this difference .
We find 27 radio galaxies to be members of A2125 , and only four in A2645 , based on ( nearly ) complete observations to the same limiting magnitude and radio flux density .
The radio galaxies in A2125 extend over about 5 Mpc ( assuming H _ { 0 } = 75 km s ^ { -1 } Mpc ^ { -1 } ) along a band running from NE to SW of the cluster center .
About half the radio galaxies are red and have optical spectra which resemble old stellar populations .
The other half are blue with emission lines , most of which indicate an origin in star formation rather than AGN .
Many of the blue galaxies are located in a distinct clump located about 2 Mpc in projection from the cluster center .

The excess population of radio galaxies in A2125 occurs entirely at radio luminosities < 10 ^ { 23 } W Hz ^ { -1 } , where one expects star-formation to be primarily responsible for the radio emission .
Most of these radio galaxies have optical properties most consistent with systems later than E/S0 .
However , the optical line luminosities are often weaker than one would expect for the star formation rates implied by the radio emission .
Thus we suspect that dust obscuration , larger than is usually found locally , hides most of the star-forming regions optically .

The existence of a cluster-cluster merger in progress in A2125 seems likely to play some role in these phenomena , although the details are obscure .

\keywords galaxies : active—galaxies : clusters : individual ( Abell 2125 , Abell 2645 ) —galaxies : distances and redshifts—galaxies : evolution—galaxies : kinematics and dynamics— galaxies : photometry—galaxies : starburst