A double immunofluorescence staining technique to locate concanavalin A (Con A) surface receptors and cytoplasmic actin in the same cell was applied to monolayer cultures of rat foetal fibroblasts during cell detachment induced by trypsin and during cell attachment to glass substratum. Con A receptors were demonstrated by fluorescein-isothiocyanate-labelled Con A (FITC-Con A) and actin by specific anti-actin antibody (AAA) traced with rhodamine-labelled goat anti-human globulin (R-AHG). Untreated, control cells had an elongated shape, Con A receptors restricted to cell margins and prominent actin filaments. After 2 min treatment with 0.001% trypsin the cells became angular with Con A receptors in clusters and actin in a diffuse or aggreagate staining pattern. Progressive cell rounding followed and this was accompanied by the development of long, thin, arborized cell processes, studded with Con A receptors and containing fine actin filaments. Complete cell rounding preceded cell detachment. The sites of detached cells were marked by fine aggregates containing Con A receptors and actin. In cells attaching to a glass substratum, actin was present in a diffusely stained or aggregate pattern in round cells, in filaments restricted to cell margins in partially spread cells and in numerous filaments in fully spread cells. Con A receptors were present in clusters in round cells, in clusters or caps in partially spread cells and in cell margins in fully spread cells. Binding of FITC-Con A to partially spread cells resulted in dissolution of the few, newly formed, actin filaments. We believe our observations are consistent with the idea that actin filaments, formed during cell attachment, contribute towards the maintenance of cell adhesion by helping in the preservation of cell shape and by anchorage of Con A receptors at points of cell attachment to the substratum.