The scoring method that will be applied in the current 12th-grade science assessment project of the National Science Foundation and the Office of Educational Research and Assessment is described. The method, "graded mark-point" scoring, is modeled after procedures developed by P. Tamir for use in the performance exercises of the Israeli Matriculation Examination. The method is codified and made suitable for the item response theory scaling procedures that will be used in the analysis and reporting of assessment results. Development of the method has also been influenced by the scoring and scaling procedures of the California Direct-Writing Assessment, now used on a mass basis. Mark-point scoring uses rating forms that specify certain main points that the student should make in responding to the exercise or term. Points are identified by an expert who also provides one-sentence descriptions of the points as documentation on the rating form. Readers study the documentation and then, guided by the statements of each point, mark student papers for quality on each point. An example of the application of mark-point scoring to an open-ended item from the Earth Sciences section of the 12th-grade science assessment prototype is presented. Two figures illustrate the text. (SLD)