In a factory in northern Sweden where 120 workers were uniformly exposed to photoactive substances 73 developed occupational facial eczema while 47 showed no reaction. The workers were examined with respect to 16 genetic marker systems: HLA, blood groups (ABO, Rh, MNSs, P, K, Le and Fy) and serum groups (Hp, Tf, Gc, Pi, Bf, C3, C4 and C6). Between reactors and nonreactors the following differences were found: (1) a significant decrease (p& < 0.05) of HLA A11 among the reactors; (2) a significant increase (&p< 0.05) of the C3 FS type among the reactors; (3) a highly significant increase (&p<0.001) of the transferrin C2 gene and of the C2 variant among the reactors. The association with Tf C2 remained significant also after correction for number of significance tests. Since transferrin (iron) is known to catalyze the formation of hydroxyl radicals we hypothesize that the Tf C2 variant is more efficient in promoting radical formation and thereby cell damage. Other results supporting the notion that transferrin C2 may be associated with an increased susceptibility to toxic damage are discussed.