A number of laboratory studies have indicated that severely retarded patients respond to operant conditioning about as well as lower organisms and normal human subjects. Ellis, Barnett, and Pryer (1960) found that defectives of the type usually labeled “untestable” (seldom included in psychological experiments because of their extremely limited skills) actually adapt readily to operant conditioning procedures and that even subjects of lowest intelligence are sensitive to changes in schedules of reinforcement. Both Orlando and Bijou (1960) and Spradlin (1962) also report that severely retarded children respond to operant conditioning procedures. In each of the studies cited above, however, the investigators found that the behavior of the retarded patients was a little less reliable or less sensitive to the controlling conditions than that of lower organisms. In other words, the evidence indicates that the recently developed principles of teaching, based on reinforcement principles, could apply to severely retarded patients but with somewhat less precision in behavioral control.