Index of the English language medical journal literature covering clinical, basic sciences, and public health. Click the "MeSH database" link on the home page to perform categorical searches using Medical Subject Headings (MeSH).
Subset of Google Web Search that specifically searches for scholarly literature, including articles, papers, theses, books and reports. Includes full-text links to Himmelfarb's collection of e-journals.
American Association for Clinical Chemistry. No references so this is consumer health information, rather than professional material, but does offer an introduction and overview of what lab tests are used for and how they are carried out.