Folate, thiamin, nicotinate, biotin, riboflavin, pantothenate, vitamins A, B6, B12, C, E, and beta-carotene were determined in: (a) eight patients before and after one plasma exchange; (b) in one patient after five consecutive treatments; (c) in three patients before and 2-8 weeks after plasmapheresis. Vitamin B12, beta-carotene, vitamin B6, and vitamins A, C, E were depressed after acute or chronic plasmapheresis. Concentrations of folate, thiamin, nicotinate, biotin, riboflavin, and pantothenate were essentially unchanged after one plasma exchange.