The glutamine requirement for the in vitro proliferation of fish cells was investigated with cell lines from four different species and three tissues: goldfish skin (GFSk-S1), Chinook salmon embryo (CHSE-214), and rainbow trout liver (RTL-W1) and spleen (RTSp-W1). With a supplement of fetal bovine serum, the basal medium, Leibovitz's L-15, without glutamine supported the proliferation of all four cell lines as well, or nearly as well, as L-15 with 2 mM glutamine. This was true over short term assays of two to four weeks and for continuous propagation. CHSE-214 also grew as well with or without 2 mM glutamine in Minimum Essential Medium with fetal bovine serum. However, when the supplement was dialyzed fetal bovine serum, CHSE-214 grew much better in L-15 without glutamine. Therefore, glutamine was not required for growth in L-15, and in fact, was inhibitory in the absence of the dialyzable fraction of serum. By contrast, glutamine appeared to be important for growth in Minimum Essential Medium. When the supplement was dialyzed fetal bovine serum, CHSE-214 grew much better in Minimum Essential Medium with 2 mM glutamine. These results suggest that the glutamine requirement for the in vitro proliferation of fish cells is conditional and depends on the basal medium and serum supplement.