A procedure in which the S- was prevented from being responded to, by electro-magnets, was used to establish color discriminations. The procedure was modified in Situation 1, to include the prevention of responses to the S+ if the S- was responded to first. The original procedure and modified procedure were used in Situation 1, with only the modified procedure being used in Situations 2 and 3. The procedure of reinforcing responses to the S+ and extinguishing responses to the S-, through nonreinforcement, was used in Situation 4. Data recorded consisted of the number of trials, the number of reinforcements, and which stimulus was first responded to. Criteria for the acquisition of a discrimination was 100 first responses to the S+. Results indicated that the modified procedure was much more effective in establishing the discriminations, than the original procedure or the procedure of reinforcing responses to the S+ and extinguishing responses to the S-. The modified procedure enhanced the establishment of stimulus control, reduced the number of errors and eliminated stereotyped responses.