The Environmental Protection Agency is investigating possible dioxin contamination at four Maryland chemical plants, according to the EPA Philadelphia regional office.

EPA Regional Director Ed Skerondis said no dioxin has been found on the four sites but the chemical may have been a byproduct produced at the plants.

The companies have been identified as American Chemate, Dynasurf, and the SMC, all in Baltimore and Miller Chemical in Harford County, according to Linda Smeyne of the state health department.

Skerondis said sites may be dropped from the list if EPA's intensive investigations reveal no major traces of the chemical.

Dioxin, one of the most toxic chemicals known, is used in herbicides like Agent Orange, which was used in the Vietnam war.

Maryland and three other northeastern states--West Virginia, Delaware and Pennsylvania--are included in the EPA's latest search for sites of dioxin contamination. An EPA official said on-site testing could begin within two weeks.

The sites to be studied are largely being drawn from a 1980 report by the EPA that led to the discovery of dioxin contamination at the former Diamond Alkali Co. plant in Newark, N.J., according to George Bochanski, a spokesman for the EPA regional office. The discovery of dioxin in Newark and at Times Beach, Mo., has led to increased interest in that report.