APPLBT.FOR is the mainframe side of the Apple Kermit bootstrap.
It sends the file APPHXL.HEX to the Apple.  You must be running the
APPLBT.BAS program on the Apple to receive it.

APPLBT.FOR is written in "new" Fortran (version 6 or later).  It looks
for the file APPHXL in your connected directory.  You should

  copy ker:apphxl.hex

to put the file where APPLBT can find it.  Once you have the APPHXL
program on your micro, you run it, it asks your for a filename.  You
should type either KER:APPAC2.HEX or KER:APPDC2.HEX (depending on which
modem your Apple has); APPHXL will issue a TYPE command for that file,
and the file will thus be sent down the line to your Apple.  APPHXL
will detect errors (since the HEX files contain checksums), but cannot
correct them as real KERMIT can.

See APPLEK.DOC for details.  
