Epithelial-mesenchymal (stromal) interactions are essential in embryonic development, organogenesis and maintenance of the differentiated state (Cunha et al., 1983, 1985). Derangement of these interactions probably underlies many malignancies and other proliferative disorders (Cunha et al., 1985). Heterotypic tissue recombinants, in which an epithelium and a mesenchyme from different organs are combined, have been particularly useful in exploring these interactions. In such recombinants, the mesenchyme may permissively support the normal prospective developmental fate of the epithelium or instructively induce it to go through a new and different morphogenetic repertoire. However, is instructively induced morphogenesis accompanied by reprogramming of cytodifferentiation? Previous results are inconclusive (Sakakura et al., 1076; Neubauer et al., 1983; Yasugi, 1984; Lacroix et ul., 1984). We have examined this question using the accessory sex organs of the mammalian reproductive tract. Rat and mouse seminal vesicles arc functionally simple. Each epithelial cell synthesizes major androgen-dependent secretory proteins, collectively the structural proteins of the copulatory plug (Higgins ei ul., 1976; Fawell & Higgins, 1986; Fawell et al., 1986). Antibody probes are available for these well-characterized tissue-specific proteins (Fawell et al., 1986). Rat and mouse seminal vesicle proteins may be distinguished immunologically (Fawell et al., 1987). Seminal vesicles from neonatal rats and mice ( 1 and 2 days post purtum) and Wolffian ducts from fetal animals ( 15 and 16 days gestation) were treated with 1% (w/v) trypsin in Dulbecco's medium at 0°C for 1-2 h to allow separation of the epithelia and mesenchymes by established methods