In a recent study we showed that the growth behavior of a hematopoietic cell line (K 562) in culture was the same when using glutamine-containing dipeptides or glutamine as substrate. In this article we study the growth behavior of different tumor cells, originating from the hematopoietic system (K 562), stomach (Kato III), pancreas (Panc 1), and breast (T 47 D), to test the biological activity as preclinical in vitro screening system. We compared L-glutamine (GLN), N-acetyl-L-glutamine (ACE-GLN), L-alanyl-L-glutamine (ALA-GLN), and glycyl-L-glutamine (GLY-GLN). Cell proliferation was measured with the incorporation of [3H] thymidine or the MTT assay (cleavage of 3-(4,5-dimethyldiazol-2-yl)-2-5-diphenyl tetrazolium bromide by mitochondria). In all investigated cell types cell growth was stimulated when using glutamine-containing dipeptides or ACE-GLN instead of a glutamine-free media (not significant for T 47 D). However, GLN or ALA-GLN was advantageous to GLY-GLN or ACE-GLN when measuring cell proliferation with the MTT-assay up to 72 hours. However, alanylglutamine does not enhance proliferation, compared with free glutamine.