46 yellow and white petrolatums were screened for their skin sensitizing properties in 3 subjects with known histories of contact sensitivity to yellow petrolatum. Patch testing revealed that all the petrolatums examined contain sensitizing substances in varying amounts. The results were analyzed with regard to origin, color, purification procedures involved in the production of pharmaceutical grade petrolatum, and pharmacopoeial requirements satisfied. Allergenicity correlated positively with these parameters, but only among petrolatums of the same brand. Petrolatum sensitivity may sometimes account for false positive patch test reactions to substances diluted in it. A petrolatum which causes no reaction on healthy skin may cause eczematous reactions on damaged skin.