When fragments of 2 dissimilar embryonic tissues are placed in contact in organ culture, cells of one fragment migrate over the surface of the second to envelop it. Holtfreter proposed that this behaviour was in response to 'tissue affinities'. He proposed that these also play important roles in the control of morphogenetic cell movement during development. The present study demonstrates that the heart fibroblast, present as a minority cell type in heart ventricle, can modify the affinity behaviour of heart tissue. The fibroblast effect appears to be mediated by a factor that can be extracted from living fibroblast monolayers by 1 M urea. The factor is a cell-surface protein since it is absent in monolayers which had been treated with trypsin prior to extraction.