As a protestant, may I participate in the Eucharist (Communion) when visiting a Catholic church? I think the title sums it up.  I'm a protestant Christian, and have many Catholic friends. When I visit their churches, is it permissible for me to participate in the Eucharist/Communion ceremony by partaking of the bread and wine?
The answer is: "more than likely not".

Catholics do allow others to take part in the Holy Communion, but they maintain tight restrictions on this.


  Guidelines for the Reception of Communion
  
  For Other Christians
  
  ... Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom we are not yet fully united are ordinarily not admitted to Holy Communion.
  
  ...Members of the Orthodox Churches, the Assyrian Church of the East, and the Polish National Catholic Church are urged to respect the discipline of their own Churches. According to Roman Catholic discipline, the Code of Canon Law does not object to the reception of communion by Christians of these Churches.
  
  For Non-Christians
  
  ...While we cannot admit them to Holy Communion, we ask them to offer their prayers for the peace and the unity of the human family.


Link to article

So, when they say "not fully united", Protestants generally fall into this category.  (I'm not sure where those other churches fall in the "Protestant-Catholic" spectrum, so I'm leaving this a bit generic.)  So, therefore, Protestants are not admitted to Holy Communion.