The standard series and 35 essential oils were tested on 450 patients with dermatitis. It was found that those simultaneously sensitive to essential oils are more frequently negative than positive to balsam of Peru. The remaining 3 balsams, however (colophony, turpentine and wood tars), are also screening agents for essential oils. All four balsams reveal sensitivity to essential oils in most of the subjects tested. In patients negative to all 4 balsams of the standard series, attempts to find an essential oil with which tests would be simultaneously frequently positive with those for other essential oils failed.