The authors studied 87 individuals from four large Amish pedigrees to determine the relationship between susceptibility to affective disorders and variation in the measures of two biochemical variables, catechol O-methyltransferase (COMT) and lithium countertransport. Assays were done in duplicate and blind to diagnostic or pedigree status. Data were analyzed separately within families as well as by pooling ill and well subjects across pedigrees. Various statistical analyses yielded no significant differences in lithium measures or COMT activity levels between members diagnosed as having a form of affective disorder and the normal members.