A boundary collocation method for estimating the stress intensity factors for a through-thickness interior crack in a plate of arbitrary geometry and arbitrary in-plane loading on the plate outer-boundary and crack surfaces is presented. The collocation is carried out on an Airy stress function which is derived from a previously given complex stress function. Various types of boundary collocation procedures are investigated. It is shown that collocation on the Airy stress function and its normal derivative gives the most accurate results. The method is applied to rectangular plates containing center cracks of arbitrary angular orientation. A number of plate edge and crack edge loading conditions are analyzed. The stress intensities calculated by this method compare very favorably with existing solutions for cases in which such solutions are available.