The effect of interactions between cardiomyocytes (CM) and cardiac fibroblasts (CF) maintained in primary culture were investigated on CF proliferation. CF cultures and CM/CF cocultures were performed from enzymatically isolated cardiac cells from Swiss mice (2–4 months old) anaesthetized with ether. Fibroblast number was evaluated by counting sarcomeric a-actinin-negative, fibronectin-positive cells. When cultivated alone, CF number per well was increased from day 2 to day 8 of culture (799 ± 248 vs. 1,684 ± 646, respectively; n = 4, P < 0.01, two-tailed paired Student’s t test) and then remained virtually constant until day 12 (1,563 ± 225, n = 4). Interestingly, there was a significant increase in CF number at the beginning of the culture in the presence of CMs. After 2 days of culture, the number of CFs was increased sevenfold per well in CM/CF cocultures (6,466 ± 1,032, n = 4) compared to CF cultures alone (799 ± 248, n = 4). Thereafter, the number of CFs in coculture with CMs was increased by 40 and 31% at day 8 and day 12, respectively. These data suggest that the presence of CMs strongly favoured fibroblast adhesion and/or proliferation at the beginning of the coculture which could result from an interaction between the two cell types. We have shown that interleukin-6 (IL-6) was involved in cardiomyocyte hypertrophy and that IL-6 secretion by CM was potentiated by CF (Fredj et al. 2005). To specify the role of IL-6 on fibroblast proliferation, the effects of IL-6 and gp130 antagonists were investigated. In CM/CF cocultures supplemented with MAB406 (an IL-6 antagonist monoclonal antibody) or AF468 (a gp130 antagonist polyclonal antibody), the number of CFs decreased by 24 ± 15% (n = 3, P < 0.05) and 74 ± 4% (n = 3, P < 0.001), respectively, compared to cultures in which these agents were absent. When cultivated in the absence of CMs, CF number was not significantly modified irrespective of the culture conditions (control medium, 2,466 ± 356 (n = 3); in the presence of anti-IL-6, 2,247 ± 351 (n = 3); in the presence of anti-gp130, 1,817 ± 317 (n = 3)). These data suggest that IL-6 could be involved in fibroblast proliferation, although the strong inhibitory effect of gp130 antagonist indicates that other cytokines of the IL-6 family could also mediate this process. These results clearly show that fibroblast proliferation is favoured by CM/CF interactions with cytokines of the IL-6 family as autocrine and/or paracrine mediators. It would be interesting to identify other cytokine(s) that may be involved and the role of these interactions on fibroblast differentiation.