The effects of adding poly(lactic acid) (PLA) to the physical strength of paper test sheets prepared from three unbleached loblolly pine kraft pulps with different amounts of lignin and an aspen bleached chemothermomechanical pulp were studied. The physical strength studies demonstrated that relatively low levels of PLA addition (0.5-4.0%) could dramatically improve the tensile and burst strength properties as a function of the amount of PLA added. Hot pressing the test sheets was shown to be an important treatment for enhancing the strength properties. An analysis of untreated and PLA-treated hot-pressed test sheets by atomic force microscopy indicated that the addition of PLA markedly altered the surface properties of the sheets. (c) 2005 Wiley Periodicals, Inc.