AbstractTwelve continuous rat tissue cultured mast cell (MC) lines were established by prolonged culture of rat peritoneal MC in the absence of added factors or feeder layers. Two of these lines, RCMC1 and RCMC2, have been briefly described previously, seven others are now also described. Both RCMC1 and RCMC2 lack a marker chromosomes present on RBL‐CA10.7 cells. All lines were found to express the phenotype of mucosal MC as defined by alcian blue‐positive and safranin 0‐negative staining, the presence of rat MC protease II and a low histamine content. When analyzed for high‐(FcϵRI) and low‐affinity (FcϵRL) receptors for IgE, the various lines yielded a variety of receptor patterns. Northern blot analysis of the RNA of RCMC1, RCMC2 and RBL‐CA10.7 revealed that all three cell lines contained the same mRNA species for the α, β and γ subunits for FcϵRI previously found in another rat basophilic leukemia cell line. Quantitation of the relative amounts of α, β and γ mRNA did not correlate with the expression of the relative amounts of FcϵRI(α) in these cells. The relative amounts of mRNA for all these subunits of RCMC2 were equal or higher than those of RCMC1, suggesting that the low expression of FcϵRI(α) on the former was a consequence of post‐transcriptional events. Analysis of a RCMC1 clone over a 6‐month period revealed changes in the expressio
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