The main arachidonic acid metabolites released into the medium by mouse splenocytes have been identified on the basis of chromatographic and spectral studies as well as by mass spectrometry of the derivatives. In the absence or presence of exogenous arachidonic acid mouse splenocytes produce mainly 12-hydroxy-5,8,10,14-eicosatetraenoic and 12,20-dihydroxy-5,8,10,14-eicosatetraenoic acids. Both products are constantly released by intact cells into surrounding media without stimulation by exogenous substrate or other modulators. For the first time it is shown that exogenously added ganglioside GM3 lactone as well as ganglioside GM3 itself can influence the arachidonic acid metabolism in splenocytes.