Two high-affinity monoclonal antibodies (ROS-1, ROS-2) have been produced to the rod outer segment phosphodiesterase (ROS PDE). These antibodies bind at different antigenic determinants. ROS-2 absorbs a subset of the total PDE activity from a detergent-solubilized retina extract, whereas ROS-1 adsorbs nearly all of the PDE activity. DEAE-cellulose chromatography separates two peaks of activity from a hypotonic extract of rod outer segments. Peak I activity is adsorbed only by ROS-1, whereas peak II activity is adsorbed by both ROS-1 and ROS-2. Both peaks of activity are activated by histone H2B and limited trypsin digestion, and both peaks of activity contain a heat-stable, trypsin-sensitive inhibitor. When analyzed by SDS gel electrophoresis, ROS-1 adsorbed a single peptide from peak I, which comigrated with phosphorylase B, whereas ROS-1 adsorbed two slightly faster migrating peptides from peak II. Histone H2B activated at least 80% of the PDE activity bound to ROS-2 but was less effective in activating the PDE bound to ROS-1. ROS-1 but not ROS-2 was an effective inhibitor of PDE activity, suggesting that ROS-1 may be a specific probe to study the effects of ROS PDE on the light response.