Posttranslational gastrin processing depends on tumor morphology. Extracellular matrices have recently been demonstrated to alter cell morphology in culture. Altered cell morphology has been associated with changes in gene transcription and translation, but it is not known whether it also affects posttranslational processing. Using tyrosine-O-sulfation as a marker of processing, we studied the effects of various substrates on biologically active gastrin (IRG) production and sulfation in gastrin-containing tumor cells (GT cells). Dispersed GT cells were plated onto different substrates and then incubated. Culture media from days 4, 7, and 28 were assayed with specific antibodies that recognize total IRG and nonsulfated IRG. Cells cultured on plastic and dried films of laminin, collagen, and Matrigel (Collaborative Research Inc., Lexington, Mass.) flattened and formed monolayers of GT cells. Cells cultured on a porous membrane and hydrated gels of collagen and Matrigel did not flatten but formed spheroids of GT cells. The monolayer cultures showed an increase in sulfation with time but a decrease in IRG production. The spheroid cultures maintained a constant level of sulfation over time and, with the exception of Matrigel (gel), also showed a decrease in IRG production. These results indicate that the level of sulfation was unchanged from that of the original tumor when cells were grown in spheroids but increased when cultured as monolayers. It appears that alteration of the cellular milieu alters colony morphology, which in turn alters gastrin processing. 