Intercellular transfer of tritium-labelled uridine nucleotides has been used to detect junctional communication between various cell types in primary culture. Epidermal keratinocytes, melanocytes and dermal fibroblasts from new-born-mouse skin, and epithelial cells from baby mouse kidney form communicating junctions in all possible homologous and heterologous combinations. This lack of detectable communication specificity between cells in primary culture contrasts with the specificity shown by some established cell lines.