We studied changes in the morphology of morula cells, a common haemocyte type in botryllid ascidians, during both the rejection reaction (occurring between contacting, genetically incompatible colonies) and fusion (occurring between compatible colonies), and in short-term cultures of haemocytes incubated with heterologous or autologous blood plasma. In both the rejection reaction and haemocyte cultures in the presence of heterologous blood plasma, we observed alterations in morula cells, consistent with a degranulation event, and their expression of molecules recognised by anti-IL-1-α- and anti-TNF-α-antibodies. Anti-cytokine-antibodies markedly reduced the extent of the in vitro cytotoxicity, when haemocytes were exposed to heterologous blood plasma. In addition, the increase in the production of nitrite ions and the decrease of the in vitro cytotoxicity by the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester, suggest the role of nitric oxide in cell death. These results provide new clues to understand the process of rejection reaction in botryllid ascidians.