Residue levels of heptachlor, heptachlor epoxide, and other chlordane compounds were determined in sera from 74 Finnish plywood workers and 52 controls. Concentrations of heptachlor epoxide in plywood workers varied from below the detection limit of 0.1 ng/g to 19.2 ng/g serum (conversion factor for SI units = 975 [1 ng/g = 0.98 microgram/l]); the mean and standard deviation were 3.2 and 3.9 ng/g, respectively. Heptachlor expoxide values in controls varied from below the detection limit to 1.2 ng/g serum. The exposure time, i.e., number of years spent working with sizings that contained heptachlor, correlated with the residue levels of heptachlor epoxide that were measured in serum samples taken from employees at two companies (p = .03). The most common chlordane compound (contaminant of heptachlor) found in the sera of plywood workers was trans-nonachlor; also found were high concentrations of heptachlor epoxide. Headache, dizziness, and eye irritation were not related to serum levels of heptachlor or chlordane compounds.