RINm5F cells, a line derived from a rat insulinoma, are frequently used as a model for studying pancreatic B cell structure and function. These transformed cells are known, however, to be different from native B cells in a number of biochemical respects. We have now compared the surface features of RIN cells and native B cells in two different ways: 1) Dispersed cells from islet obtained from adult rats can reassociate spontaneously in culture to form aggregates (pseudoislets) with cellular organization similar to that of intact native islets (a central B cell core surrounded by a discontinuous mantle of non-B cells). Native islet cells and RIN cells were mixed together and allowed to reaggregate. Examination by immunocytochemistry and transmission electron microscopy showed that the aggregates contained all cell types present in the original mixed cell suspension (native B- and non-B cells, and RIN cells). The native B cells were centrally located, surrounded by zones of non-B cells, as in islets. The RIN cells, however, were restricted to the periphery and as such not recognized as native B cells. 2) R2D6, a monoclonal antibody, binds selectively to a ganglioside on the surface of islet B, but not non-B, cells. The role of this ganglioside is not known. RIN cells were incubated with R2D6 followed by a fluorescently labeled second antibody. Analysis by flow cytofluorometry indicated that the monoclonal antibody had bound (stained) only 3-15% of the RIN cells. These R2D6 positive cells were sorted from R2D6 negative cells and subsequently shown to have a lower DNA content. Expression of the R2D6 target ganglioside on RIN cells thus appears to be cell cycle dependent. Based on two different criteria, RINm5F cells do not therefore share surface features in common with native B cells. The cell cycle dependent expression of a B cell surface antigen by the RIN cells might, however, be a useful model for studying the regulation of ganglioside turnover.