The Proteomics Standards Research Group (sPRG) has initiated a study that focuses on development of a standard that can be used in both assessment of a laboratory's ability to detect an array of post-translational modifications in a complex proteomic sample and development of new approaches for characterization of post-translationally modified proteins. The sample that has been generated for the first stage of this study contains a mixture of more than seventy synthetic peptides with a variety of modifications, in a tryptic digest of six proteins. Modifications represented in the sample include acetylation, methylation, nitration, phosphorylation, and sulfation. The individual proteins were purified, digested and analyzed prior to formulation of the sample. The synthetic peptides were each analyzed and then mixed with the digests; the mixture was aliquoted and lyophilized in sufficient quantities that would permit evaluation by several analytical approaches. The sample was fully characterized by members of the sPRG. It is planned that in 2011, the sample will be distributed to requesting laboratories for a larger study focusing on PTM characterization. The sPRG will present results obtained at different stages of preparation and characterization of the sample. Approaches for analysis of complex proteomic samples containing various post-translationally modified proteins will also be discussed. The sPRG is planning to open the study for sample requests at the ABRF 2011 conference. From the responses returned by the participating laboratories, the sPRG will subsequently report on the effectiveness of the approaches used for characterization of posttranslational modifications in protein digests.