Education is an effective tool for modifying physician use of the laboratory. We compared two interventions by assigning 56 medical house officers into four groups: control group; feedback group, which received feedback concerning its use of tests; manual group, which received a manual concerning cost-effective laboratory use; and manual plus feedback group, which received both interventions. All intervention groups experienced significant decreases in test use. When we controlled for diagnosis, the manual plus feedback group had the most profound decrease (42%) in laboratory use, followed by the manual group. The feedback and control groups had no change. Attitudes and knowledge did not change. We conclude that one can, via simple techniques, modify house staff use of the outpatient laboratory. The less-expensive intervention was a cost-oriented manual, which may have a "sensitizing" rather than educational effect.