The key enzyme of ethylene biosynthesis, ACC synthase, is encoded by a multigene family. We describe three new DNA sequences encoding members of the ACC synthase family of the tomato. One of these sequences encodes a novel ACC synthase, LE-ACS6, which is phylogenetically related to the ACC synthases LE-ACS1A and LE-ACS1B. Gene-specific probes for seven tomato ACC synthase genes were prepared. They were used for RNase protection assays to study the accumulation of ACC synthase transcripts in suspension-cultured tomato cells after the addition of an elicitor. The ACC synthase genes LE-ACS2, LE-ACS5 and LE-ACS6 were strongly induced by the elicitor. In contrast, the genes LE-ACS1B, LE-ACS3 and LE-ACS4 were constitutively expressed and LE-ACS1B was present at all times at a particularly high level. Thus, there are two groups of ACC synthase transcripts expressed in these cells, either elicitor-induced or constitutive. A transcript of LE-ACS1A was not detected. Despite the presence of LE-ACS1B, LE-ACS2, LE-ACS3, LE-ACS4 and LE-ACS5, there was only little ethylene produced in the absence of the elicitor. Increased ethylene production is usually correlated with the accumulation of ACC synthase transcripts, indicating that ethylene production is controlled via the transcriptional activation of ACC synthase genes. However, the abundance of several ACC synthase mRNAs studied was not strictly correlated with the rate of elicitor-induced ethylene production. Our data provide evidence that the activity of these ACC synthases may not solely be controlled by the transcriptional activation of ACC synthase genes.