In contrast to established cell lines, normal human skin fibroblasts spread on their own fibronectin. The present investigation has examined whether human fibroblasts, like established cell lines, would be capable of spreading on substrata coated with proteins with different reactivity towards the cell surface. Coverslips were coated with human serum, fibronectin, alpha 2 macroglobulin-trypsin, a polyspecific anti-fibroblast antibody and a polyspecific anti-calf-serum antibody. The attachment of the cells to these substrate was of the same extent. Spreading was examined qualitatively using phase, interference contrast and reflection contrast optics on live cells, as well as scanning electron microscopy on fixed cells. To quantitate the maximum degree of cell spreading a semi-automated system was used, which measured the cell perimeter on a large number of cells. The distributions of the degree of cell spreading on the five substrata were compared statistically. The qualitative and quantitative differences observed on the various substrata could be further differentiated by adding various amines to the cells during 60 min spreading or during a 30 min preincubation before spreading. No strict correlation could be found between the effect of the amines on attachment or on spreading and their presumed effects on cellular transglutaminases. The results clearly indicate that the spreading of human fibroblasts can be modulated by the nature of the substratum and that, by using quantitative methods, these differences in behaviour can be measured accurately.