Quantitative data are presented regarding cell proliferation and migration on (a) collagen films (b) the surface of 3-dimensional gels of native collagen fibres and (c) within the 3-dimensional collagen gel matrix, as part of a study of the effects of the extracellular matrix on cell behaviour. The nature of the collagen environment was found to influence the proliferation of certain cell types, but not of others. For example, HeLa cells proliferate at approximately the same rate and reach the same saturation cell densities on all of the collagen substrata, while human skin fibroblasts grow more slowly within the 3-dimensional collagen gel matrix compared with cells either on the gel surface or on collagen films. The 3-dimensional gels of native collagen fibres may also be used to study cell migration on the gel surface, as well as cell migration (or 'infiltration') from the gel surface into the 3-dimensional collagen matrix. Two methods have been used to obtain quantitative information concerning cell infiltration into the collagen gel, one involving the selective removal of cells from the gel surface, while the other relies on direct microscopic examination. Of the cells examined to date, epithelial cells (both normal and tumour) do not show infiltrative behaviour, while both normal and virally transformed fibroblasts, as well as tumour cells of non-epithelial origin (e.g. melanoma), do infiltrate into the collagen gel matrix, at rates which vary considerably according to cell type.