Arabinogalactan proteins (AGPs), a family of hydroxyproline-rich glycoproteins, occur throughout the plant kingdom. The lysine-rich classical AGP subfamily in Arabidopsis consists of three members, AtAGP17, 18 and 19. In this study, AtAGP19 was examined in terms of its gene expression pattern and function. AtAGP19 mRNA was abundant in stems, with moderate levels in flowers and roots and low levels in leaves. AtAGP19 promoter-controlled GUS activity was high in the vasculature of leaves, roots, stems and flowers, as well as styles and siliques. A null T-DNA knockout mutant of AtAGP19 was obtained and compared to wild-type (WT) plants. The atagp19 mutant had: (i) smaller, rounder and flatter rosette leaves, (ii) lighter-green leaves containing less chlorophyll, (iii) delayed growth, (iv) shorter hypocotyls and inflorescence stems, and (v) fewer siliques and less seed production. Several abnormalities in cell size, number, shape and packing were also observed in the mutant. Complementation of this pleiotropic mutant with the WT AtAGP19 gene restored the WT phenotypes and confirmed that AtAGP19 functions in various aspects of plant growth and development, including cell division and expansion, leaf development and reproduction.