A proteinaceous fiber is a long, thin strand present in the matrix of a connective tissue. The fiber is made of at least one protein and possesses a string-like arrangement; it is submerged in the ground substance to prevent it from separating from the tissue.
Proteinaceous fibers can be made of many proteins. The most common fibers are collagenous and/or elastic; accordingly, they are made of collagen and/or elastin, resulting in their descriptors.