TPS has been initiated from a long-term pronephric stroma culture of an adult 1 year old rainbow trout (Oncorhynchus mykiss W). The cells grow at 4 degrees to 22 degrees C, with an optimum temperature of 18 degrees C. They do not form true monolayers, but overlap in a multilayered network of fibroblastic, epithelioid and giant cells. TPS cells are susceptible to viral haemorrhagic septicemia virus (VHSV) and infectious haemopoietic necrosis virus (IHNV). They have been applied to support the development of haemopoiesis when co-cultured with haemopoietic precursors. Spontaneous angiogenesis processes in vitro occurred in TPS cultures when they are kept in the same flask, i.e. without passages, for long times (over 2 months).