Exploratory protein analysis in mid-trimester maternal sera by 2-dimensional electrophoresis and image analysis was performed to determine the differences between mothers carrying fetuses with Down syndrome (DS) and control mothers. Nine haptoglobin alpha-1 and alpha-2 (Hp1 and Hp2) variants were detected. Three apparent isoforms of the Hp2FF protein having the same relative charge but different Mr's were detected, with the two smaller variants--Hp2FF (-0.7k) and Hp2FF (-1.3k)--believed to be first reported here. Ninety-two percent of the cases (n = 83) had the Hp2FF (-0.7k) protein, and 61% had Hp2FF (-1.3k). The geometric mean concentration of Hp2FF (Mr = 18.0 kd) was found to be significantly increased (P less than 0.01) in mid-trimester sera from cases (n = 27) compared to controls (n = 56). When the concentrations of the three Hp2FF isoforms were added, the cases had larger sums on average than controls (P less than 0.01).