It is becoming increasingly apparent that interferon exerts a major role in cellular regulatory processes. Besides its well known antiviral properties, interferon has been shown to affect cell surface antigen expression, lymphoid cell function, as well as cellular protein synthesis and growth. To date, however, there is little information available on the mechanism of interferon production in vivo except in response to a variety of exogenously administered viral and nonviral systemic inducers. We report here the detection of a pregnancy-associated elevation of interferon, or an interferon-like component, in selected reproductive tissues of mice.