This paper describes a procedure for establishing primary cultures of endocardial endothelial cells and morphologically characterizes the cultivated endothelial cells from Atlantic cod (Gadus morhua L.) and salmon (Salmo salar L.). Following incubation with collagenase and trypsin, the resulting cell suspension was seeded on substrates of fibronectin, collagen or gelatin. Cod and salmon endothelial cells attached firmly to these substrates within 24 and 5 h respectively, but did not attach to uncoated tissue-culture plastic. The contamination by non-endothelial cells was kept at a minimum by working out optimal combinations of serum concentration and growth substrate. The yield was about 3x10(6) and 1x10(6) cells per kg cod and salmon, respectively. Cultures could be maintained for several weeks but cells did not proliferate. Cultivated cod endocardial endothelial cells differed from salmon endocardial cells in having abundant cytoplasm with many vesicles and avidly taking up labelled collagen. The described method allows, for the first time, functional and morphological studies to be performed under controlled conditions on both a specialized scavenger type of endothelium from cod and a conventional flat endothelium from salmon.