Colophony is a complex mixture of over 100 compounds derived from pine trees. It has countless applications at home and at work and exposure to colophony and modified-colophony is universal. It is the oxidation products of unmodified and modified colophony and some of the new resin acids synthesized during modification that are the principle allergens in colophony. The neutral fraction may account for a small % of positive reactions. When screening for allergy using unmodified gum rosin, allergy to modified rosin will not be revealed. When patients react to both materials, it is probably due to unmodified colophony present in both, rather than a cross-reaction. Relevant positive reactions may be missed if only colophony 20% pet is relied upon as the screening material.