Experiments were carried out with the suckling mouse cataract agent (SMCA) in an attempt to better characterize the nature of this agent. Despite the fact that high titers of SMCA were found in the chorioallantoic fluid of inoculated chicken embryos, the agent did not replicate in chicken embryo epithelial cell cultures. SMCA did not interfere with Sindbis virus plaque formation and was not inhibited by chicken interferon or polyinosinic-polycytidylic acid. The agent was relatively resistant to high pH and osmotic shock but was inactivated by low pH.