We have compared the behaviour and the ultrastructure of embryonic mouse fibroblasts embedded into collagen gels at early, middle and late population doubling levels (PDL). Late mouse fibroblasts were able to induce gel contraction with a greater efficiency than young cells. We did not find any differences in the organization of these gels. Electron microscope observations on gels containing fibroblasts of different PDL showed that the collagen lattice induced new specific and distinct phenotypes. The well-known ultrastructural differences between young and late fibroblasts grown on plastic substrates were less prominent when these cells were embedded into collagen gels. The late fibroblasts grown into gels kept their large size and their lobulated nuclei and resembled fibroblasts grown on plastic surfaces. However, dramatic changes were observed in their pattern of microfilaments, in the dispersion of their chromatin and in their ergastoplasmic structure; these characteristics observed in late fibroblasts grown into gels were close to those of young cells. The new phenotypes of young, middle-aged and late fibroblasts in the collagen gels seemed to be stable and did not display the characteristics of an older phenotype on continued incubation. When the fibroblasts left the gel, they returned to their initial phenotype.