one of their causes was the United  Farm Workers Movement.
[youtube=http://www.youtube.com/watch?v=8swg1bo0J6c]
from the nytimes 1998:
Thirty years ago, the folk singers Peter, Paul and Mary performed in a Carnegie Hall benefit concert to support the United Farm Workers' effort to seek higher wages for California grape workers. On Thursday, the trio will celebrate the anniversary by once more singing for the union. This time they will be a lot closer to the fields. The show, at the Santa Cruz Civic Auditorium in California, is a benefit for the union's effort to organize strawberry pickers in the region.