Morbidity and mortality associated with ineffective drugs can be avoided. Drug resistance induced by ineffective treatment is avoided. Relative efficacy of two similar regimens is elucidated. Rational treatment choices are possible when no clearly defined treatment protocols exist. When standard regimens fail, newer or non-standard. regimens can be considered A therapeutic index can be obtained when normal cells are also tested. Financial savings ensure as responding patients need less supportive therapy. Unnecessary use of expensive new drugs can be avoided in test-resistant patients.