Marjorie Janice Groothuis Horning (August 23, 1917 – June 11, 2020) was an American biochemist and pharmacologist. She was considered to be a pioneer of chromatography for her work in developing new techniques and applying them to the study of drug metabolism. She demonstrated that drugs and their metabolites can be transferred from a pregnant woman to her developing child, and later through breast milk, from a mother to a baby. Horning's work made possible the prevention of birth defects, as doctors began to warn of the dangers of drugs, alcohol, and smoking during pregnancy.