A proteomic approach was applied to compare the secretome (culture filtrate proteome) of phosphate-sufficient (+Pi) and Pi-deficient (-Pi) Arabidopsis thaliana suspension cell cultures. Secretomes harvested from the +Pi and -Pi cells yielded dissimilar 2-DE maps. PMF via MALDI-TOF MS resulted in the identification of 50 protein spots representing 37 discrete proteins having unique gene identities. A total of 24 Pi-starvation responsive proteins were identified, with 18 of these being up-regulated and six down-regulated. Secreted proteins up-regulated by the -Pi cells included a ribonuclease involved in Pi scavenging from extracellular nucleic acids, as well as enzymes of cell wall modification, proteolysis, pathogen responses, and ROS metabolism. Enzyme activity assays and immunoblotting demonstrated that a pair of purple acid phosphatase isoforms having subunit M(r)s of 65 and 55 kDa was also secreted by the -Pi cells. Semiquantitative RT-PCR was used to assess the relationship between mRNA levels and relative amounts of selected secretome proteins. The results indicate that transcriptional control is but one of many factors contributing to Arabidopsis Pi starvation responses, and highlight the importance of parallel biochemical/proteomic studies of -Pi plants.