Eight day (8-d CEF) and 16 day old chick embryo fibroblasts (16-d CEF) obtained after a mild trypsin treatment (50 micrograms/ml in Ca2+ and Mg2+-free PBS, plus 10 mM EDTA) for 10 min at 37 degrees C present the same number of fibronectin (FN) binding sites at their surface (approximately 550,000 sites per cell) with a Kd approximately equal to 1.40 microM in both cases. Furthermore, FN interacted with high molecular weight plasma membrane proteins (150,000 and 125,000) insensitive to trypsin treatment. Both 8-d and 16-d CEF adhered and spread to the same extent on a fibronectin coated substratum (80% of the CEF adhered in 60 min). In contrast, 8-d and 16-d CEF behaved differently towards laminin (LM). 8-d CEF exhibited approximately 5500 binding sites per cell with a Kd of 1.5 nM (Codogno P., Doyennette, M.-A. and Aubery M., 1987, Experimental Cell Research, 169, 478-489.) and were highly sensitive to trypsin treatment, whereas 16-d CEF do not express cell surface binding sites for laminin. Differences were also observed in the adhesive capacities of 8-d and 16-d CEF on LM substrata: 8-d CEF adhered and spread on LM in a very specific manner (60% of the cells adhere in 60 min) and 16-d CEF did not adhere to LM even after long periods of incubation exceeding 360 min.