News release: “Creative Commons (CC) licenses are types of copyright licenses that allow free distribution of a work. PubMed Central (PMC), the NLM archive of full text journal articles, includes articles that have a Creative Commons license or are in the public domain. The latter covers those articles authored by staff of U.S. government agencies. NLM now offers a filter for both PubMed Central and PubMed to find articles with unrestricted Creative Commons or public domain (CC0) licenses:

PubMed Central: http://www.ncbi.nlm.nih.gov/pmc/?term=cc0+license%5Bfilter%5D or add cc0 license[filter] to your search
PubMed: http://www.ncbi.nlm.nih.gov/pubmed/?term=pmc+cc0+license%5Bfilter%5D or add pmc cc0 license[filter] to your search

These filters are based on license information that is provided to PubMed Central by publishers. There are many more articles cited in PubMed but not deposited in PMC that have CC0 licenses, but that information is not part of PubMed.”