A technique for quantitative determination of human placental lactogen (HPL) hormone and alpha-foetoprotein (AFP) in serum or plasma in the same rocket immunoelectrophoresis is described. The HPL rockets are stained with Coomassie brilliant blue and the AFP rockets are visualized by auto-radiography or by a spark chamber technique. Sixty-seven HPL-AFP estimates were made on different sera from 21 women with twin pregnancies. Both the HPL and AFP concentrations were higher in serum from twin pregnancies than in serum from single pregnancies. The diagnostic use of the combined HPL-AFP assay, especially in the early diagnosis of twin pregnancies and of neural tube malformations, is discussed.