Check out the CLASS clause in the SPECIAL-Names pgraph of the CONFIGURATION SECTION of the ENVIRONMENT DIVISION.
You can create a CLASS that contains all the chars that you deem acceptable. Then, in the proc div you can code a CLASS test to determine that your 32 byte field only contains those acceptable chars.
Example: Define a CLASS as "ACCEPTABLE"
In the proc div, code:
IF FLD-32 IS ACCEPTABLE
One cautionary note. When using the THRU feature in defining the CLASS, remember that 'A' THRU 'Z' defines all chars with bit configurations between X'C1' and X'E9'. That means that there are gaps between "I" and "J", "R" and "S" that contain values that are not alphabetic.
Take a look at Appendix B of the COBOL Reference Manual. It contains the EBCDIC & ASCII collating sequences, you'll see what I mean.
Use the PROC DIV code I listed in a previous post. The IF stmt will PERFORM the code that will process the field if it ONLY contains the chars listed above. If your testing shows that the list doesn't contain ALL the acceptable chars just expand the list.