In the current study two monoclonal antibodies (mAb) were used to investigate the expression of adult acinar and duct cell-specific antigens and their relationship with cell growth in primary acinar cell cultures. We have previously found that adult mouse pancreatic acinar cells divide in primary culture. Furthermore, during growth the cells lose their differentiated morphology and exhibit decreased expression of secretory proteins, followed by some degree of morphological redifferentiation after reaching confluency. A mAb specific in the adult pancreas for acinar cells (mAb Acinar-1) and another specific in the adult pancreas for duct cells (mAb Duct-1) were generated using such cultures as the immunogen. The starting material for the cultures consisted of predominantly Acinar-1 positive cells which incorporated [3H] thymidine, as determined by autoradiography and immunofluorescence labeling. However, expression of the acinar antigen persisted for only the first 3 to 7 days in culture. By contrast, expression of the duct antigen was rare until after 5 days in culture and was highest at day 9, the peak of cell growth. Dual label immunofluorescence showed that during the growth phase fewer cells expressed the acinar antigen, most expressed the duct antigen, and occasional cells expressed both antigens. After reaching confluency, the growth rate declined from days 15 to 21, and the cells progressively regained the acinar antigen with a concomitant loss of the duct antigen. mAb labeling was morphometrically quantitated and showed that more than 97% of the labeled area was Acinar-1 positive at 3 days, which decreased to approximately 16% at day 9, and then returned to over 97% by day 21 of culture. Ultrastructural immunolabeling showed that Acinar-1 positive cells at 21 days had well organized rough endoplasmic reticulum and small apical vesicles, while Duct-1 positive cells were undifferentiated in appearance (day 9) or had numerous mitochondria (day 21). Thus, changes in cell-specific antigens were paralleled by cell type associated morphological characteristics and indicate that adult acinar cells can retrodifferentiate to a more duct-like cell while retaining the potential to express an acinar-specific antigen.