If an application opens an ISAM data set, the system prints informational messages IEC134I and IEC143I to the job log. The IEC134I message allows you to identify applications that use ISAM data sets so that you can convert the data sets to VSAM. The system programmer can automate detection of IEC134I:

IEC134I jjj,sss,dsn Z/OS AS OF V1R7 DOES NOT ALLOW OPENING OF INDEXED SEQUENTIAL
DATA SETS

IECI43I 213-1C,mod,jjj,sss,ddname[–#],dev,ser,dsname 

IBM provides the ISAM compatibility interface that allows you to run an ISAM program against a VSAM key-sequenced data set. To convert your ISAM data sets to VSAM, use the ISAM compatibility interface or IDCAMS REPRO.

Related reading: For more information, see Using ISAM Programs with VSAM Data Sets and z/OS Migration.