Improved maintenance in vitro of the hematopoietic tissue of the Dublin Bay prawn Nephrops norvegicus (L.) resulted by using 10% (v/v) 2x Leibovitz's medium prepared in seawater (salinity = 25 per thousand), and supplemented with 10% (v/v) heat inactivated fetal bovine serum plus 5% (v/v) Nephrops serum or 5% (v/v) Nephrops muscle extract, and 0.06 g/l of L-proline and 1 g/l of glucose. Pronase at 100 microg/ml improved tissue dissociation and subsequent spreading of hematopoietic cell cultures. The addition of epithelial growth factor (EGF), based fibroblast growth factor (bFGF) or insulin growth factor 1 (IGF-I) did not enhance cell growth. Cell culture contained several types of maturing hemocytes, in the size range of 6-24 microm diameter. Acid phosphatase, alpha-naphthyl butyrate esterase, alpha-naphthyl acetate esterase, naphthyl AS-D chloroacetate esterase activity and phenoloxidase activity was demonstrated, but not so alkaline phosphatase or peroxidase. Small PAS (= Periodic acid Schiff) positive granules, unsaturated lipids and phospholipids were observed. Cultures remained functional for over two weeks. Mitosis was noticed occasionally; however, cell proliferation was not recorded by use of nuclear proliferation markers.