The experiences of course teams in developmental testing for credit between 1976-79 are described. The term "developmental testing," as used at the Open University, is defined as a stage in the course development process in which draft learning materials are tried out on students (or surrogate students) to obtain feedback before implementation. The place of developmental testing in the Open University's course development process is discussed, and developmental testing is compared with course improvement models that involve collecting feedback from students shortly after presentation. A rationale for the choice of testing for credit rather than testing for payment is also considered. Four case studies describing various aspects of the developmental testing of four courses (A241--elements of music, A292--Greece 478-336 BC, A101--arts foundation, and U20l--risk) are presented, along with a note on the testing of another (S337--earth science). Each case study considers a number of issues, but each focuses primarily on a different aspect of developmental testing. Based on the case studies, the following conclusions about developmental testing for credit are offered: (1) this procedure does not, in genera1, lead to major structural revision of a course, and only incidentally to content changes, although it does result in a wide variety of modifications to the teaching of a course; (2) it can lead to benefits for students; (3) it can lead to benefits for authors; and (4) its costs and benefits are difficult to quantify, but the costs do not appear excessive. (Author/SW)