Agricultural workers are encouraged to wear sunscreen to reduce their risk of skin cancer. These workers are also exposed to herbicides during the course of their day. The skin is the major source of chemical exposure in agriculture. The purpose of this work is to determine the effect of sunscreen use on the transdermal absorption of a model herbicide, 2,4-dichlorophenoxyacetic acid. Hairless mouse skin was pretreated with one of nine commercially available sunscreens purchased at a local drug store. The herbicide 2,4-dichlorophenoxyacetic acid was placed on top of the epidermis in an in vitro diffusion chamber for 24 hours. The total penetrating through the skin in 24 hours ranged from 39.1 +/- 1.7% for the no sunscreen control to 81.0 +/- 2.8% for Neutrogena Oil Free Sunscreen. Of the nine sunscreens tested, six led to a significant enhancement of total 2,4-dichlorophenoxyacetic acid penetration as compared to the control (p < 0.01). Careful selection of sunscreen during pesticide application could reduce potential exposure.