Trouble with NFS access to UNIX box

[Healy@space50.nrl.navy.mil (Liam M. Healy)]

I am having trouble getting NFS to work between the Symbolics (under
8.1) and our UNIX box Mercedes (a Sun 3/50 running SunOS 4.1.1).  No
matter what I try I always am denied access to files on Mercedes, either
with

No UNIX passwd information found for user name "HEALY" on host MERCEDES.
Enter user name for host MERCEDES, or type Return to login anonymously: 

or a box stating "Permission Denied" and offering multiple choices.  I
have tried using several usernames all valid on mercedes, but no dice.  According
to the Genera 8.1 Bundled Products Documentation (sec. 3.2.3) there are
three mechanisms used for UID/GID mapping attempted in order
1) Read /etc/passwd, /etc/group  -- all users I have tried are on these files
2) Use Sun NIS  -- we're not running NIS aka Yellow Pages
3) Consult namespace database -- I tried putting entries in here, makes no difference.
I believe I have done all the right things on Mercedes -- 
put directories in/etc/exports, run exportfs, reboot etc.

Any clues?
Thanks.

================================================================================
Liam M. Healy 
Code 8242, Naval Research Laboratory, Washington, DC 20375
Telephone: 202-404-8338 
Internet: Healy@space50.nrl.navy.mil 
Composed on 11/18/91 13:38:13 at SPACE20 (a Symbolics 3620)