The purpose of the present study is to develop a simple tool to measure interactive learning in the context of computer-mediated communication (CMC). In this article, the description of such measure of interactivity, namely the Measure of Interactivity in CMC (MICMC) is provided, and the reliability and validity of this measure are examined. In addition, several interesting relationships observed during the development of MICMC are discussed. The subjects for this study are selected from students who participated in the Knowledge Forum Knowledge Building Community Project in 1999. It was a fifth- and sixth-grade classroom in an inner city elementary school with 23 students. The analysis of the data indicates that MICMC is a useful measure of interactivity. It goes beyond how much time people spend using CMC, how many messages are created, or even how many students are involved. Instead of being a measure of participation, it becomes a measure of collaboration in terms of interactivity.