A cell line, RIN-m, established from a transplantable rat islet cell tumor secretes insulin (IRI) and somatostatin (SRIF) and expresses high levels of the key amine precursor uptake and decarboxylation (APUD) cell enzyme L-dopadecarboxylase (DDC). Conditioned medium from a rat pituitary tumor line GH3, secreting GH and PRL, improved the cloning efficiency of RIN-m cells 24-fold and enabled the isolation and establishment of a large number of primary and secondary clones. These clones were used to study clonal relationships between peptide hormone secretion and APUD features of an endocrine cell. All the primary and secondary clonal derivatives, irrespective of whether they secreted peptide hormones, maintained high levels of DDC activity. In contrast, IRI and SRIF secretion patterns of the primary clones were highly variable. Selective recloning of primary clones resulted in the isolation of subclones which produced either no hormones or high levels of either IRI or SRIF, but no clone that continuously secreted high levels of both IRI and SRIF. We conclude that: 1) the ratpituitary tumor line GH3 produces a factor(s), possibly GH and/ or PRL, which dramatically affects the growth and cloning efficiency of rat islet tumor cells; 2) in contrast to the variability in hormone secretion patterns, DDC activity was consistently expressed in all clones and subclones; and 3) although wide fluctuation in hormone secretion levels occurred among the primary clones, subclones were obtained which revealed that IRI and SRIF can be expressed independently. The subclones of RIN-m developed should be useful for the analyses of factors influencing the synthesis, storage, and secretion of IRI and SRIF. The persistence of high DDC activity in the primary andsecondary clones suggests that the APUD property of this endocrine cell may be a primitive differentiation feature closely related to the stem cell; in contrast, peptide hormone production may be associated with more terminal differentiation events.