The effects of computer-based diagnostic testing on the laboratory achievement of 91 preservice elementary teachers were assessed. These teachers were enrolled in one of four laboratory sections of a general science course. Intact classes were randomly assigned to one of two treatment groups. All students completed the same laboratory activities during a 7-week period. Students in group one (diagnostic group) were then provided with access to a 22-item, multiple-choice, computer-based diagnostic test. Upon completion of the test, a summary of the subjects' performance on various course objectives was provided. Students in group two (non-diagnostic) did not have access to the diagnostic test. Following treatment, all subjects took a teacher-made laboratory examination. Subjects in the diagnostic group also completed an instrument to determine their attitudes toward the usefulness of the diagnostic test. Results indicate that students in the diagnostic group did not perform significantly better than students in the non-diagnostic group on the achievement test. Approximately one-half of the subjects in the diagnostic group reported that the test could be improved by providing answers to the questions that they missed. Results of the attitude survey suggest that students believe the diagnostic test was helpful in preparing for the laboratory examination. (Author/JN)