We have compared the levels of the integral plasma membrane glycoprotein CE9 (MRC OX-47) in different tissues of the rat and have ascertained that the levels of CE9 protein and mRNA in selected tissues and cells exhibit moderate increases in response to diverse stimuli of metabolic activation. When normalized on the basis of total protein, the level of CE9 detected in the different tissues was found to vary over a 50-fold range. In addition, the apparent molecular mass of CE9 was observed to vary from 40 kDa to 68 kDa as a consequence of tissue-specific glycosylation. The highest level of CE9 was detected in brown adipose tissue, where the protein was found to be localized to the plasma membranes of the adipocytes. The metabolic activation of brown adipose tissue that occurs upon exposure of rats to the cold was found to be accompanied by 3.0 +/- 0.4-fold and 1.7 +/- 0.2-fold increases in the levels of CE9 mRNA and protein respectively. An intermediate level of CE9 was detected in the liver, where the protein is known to be expressed within the basolateral domain of the hepatocyte plasma membrane. The metabolic activation of hepatocytes that occurs upon administration of thyroid hormone to euthyroid rats was found to be accompanied by 2.2 +/- 0.3-fold and 1.9 +/- 0.3-fold increases in the levels of CE9 mRNA and protein respectively. A low level of CE9 was detected in the lymphoid organs, such as thymus and spleen. The metabolic activation of isolated rat splenocytes that occurs upon concanavalin A-mediated blast transformation in culture was found to be accompanied by 2.1 +/- 0.2-fold and 1.6 +/- 0.2-fold increases in the levels of CE9 mRNA and protein respectively. On the basis of these and other observations, we suggest that the level, and possibly also the localization, of the integral plasma membrane glycoprotein CE9 may be correlated in a positive fashion with metabolic activity in a diverse array of cell types.