Conventional mastery tests designed to make optimal mastery classifications were compared with fixed-length and variable-length adaptive mastery tests. Comparisons between the testing procedures were made across five content areas in an introductory biology course from tests administered to volunteers. The criterion was the student's standing in the course, based on examinations and laboratory grades. Results showed adaptive tests resulted in mastery classifications more consistent with final class standing than those obtained from conventional test. This result was observed within individual content areas and for discriminant analysis classifications made across content areas. This result was also observed for two scoring procedures used with the conventional tests. Results indicated that there was no decrement in the performance of the adaptive test when a variable termination rule was implemented. Further analyses shows that the adaptive tests administered differed from the conventional test for each content area as a function of achievement level. This evidence was used to explain why the adaptive tests resulted in more valid decisions than the conventional procedure. Variable-length adaptive mastery tests can provide more valid mastery classifications than "optimal" conventional mastery tests while reducing test length an average of 80% from the length of conventional tests. (Author)