In this study the effect of pentachloronitrobenzene (PCNB) on fibronectin (FN) distribution on the early stages of White Leghorn chick embryos was investigated by using immunohistochemical-staining method. PCNB was dissolved in vegetable oil and two different PCNB solutions of 10 and 100 fold of the daily acceptable dose was applied to fertilized chick eggs. After incubation, sections from the desired stages of embryos were taken and stained.