Ultradian protein synthesis rhythm was used as a marker of cell cooperation in synchronous dense and non-synchronous sparse hepatocyte cultures. Phenylephrine (2 μM, 2 min), an α 1-adrenoreceptor agonist, which exerts [Ca2+]cytelevation from intracellular stores, affected protein synthesis rhythm in sparse cultures, i.e. initiated cooperative activity of the cells. The same effect was produced by 2,5-di(tertiary-butyl)-1,4-benzohydroquinone (10 μM, 2 min), which increases [Ca2+]cytby a non-receptor pathway. Pretreatment of dense cultures with the intracellular calcium chelator, 1,2-bis (2-aminophenoxy) ethane-N,N,N′,N′- tetraacetic acid (acetoxymethyl) ester (BAPTA-AM) at 10–20 μM for, 30–60 min resulted in loss of the rhythm of protein synthesis, i.e. loss of cooperative activity between the cells. The medium conditioned by control dense cultures initiated rhythm in sparse cultures, whereas the conditioned medium of cultures pretreated with BAPTA-AM did not. [Ca2+]cytincrease is known to occur with monosialoganglioside GM1 treatment. By ELISA estimation, the GM1 content in 3 h conditioned medium was similar in control dense cultures to that in cultures pretreated with BAPTA-AM. Bearing in mind data on the Ca2+-dependence of vesicle formation and shedding, the conditioned medium was separated by 150,000 g centrifugation to supernatant containing monomers and micelles, and a pellet containing vesicular form of gangliosides. Only the latter initiated cooperative activity of the cells of sparse cultures. These cultures were also synchronized by GM1-containing liposomes at lower concentrations than added free GM1, 0.0003 and 0.06 μM respectively. Thus, GM1 and calcium are both involved in cell—cell synchronization. Activation of gangliosides, including GM1 and elevation of [Ca2+]cyt, is known to lead to changes of protein kinase activity and protein phosphorylation resulting in modulation of oscillations in protein metabolism.