We have investigated the expression of oestrogen receptor (ER) mRNA in Ficoll-separated tonsillar cells and the changes that occur with the addition of oestradiol (OE2) both in the presence and the absence of the T cell mitogen phytohaemagglutinin (PHA). The amounts of ER mRNA and β-actin mRNA in the samples were determined by slot blotting and hybridization and quantified by densitometry. The levels of ER mRNA were normalized against the β-actin mRNA content. In the presence of OE2 (7×10−8 m) after a 10-h culture there was a significant decrease (P<0·05) to about 66% of the control (0-h culture) ER mRNA levels. Stimulating the cultures with PHA (1 μg/ml, without the presence of OE2, had no effect on the expression of ER mRNA. However, when OE2 was present in a 10-h culture of PHA-stimulated cells, the ER mRNA level was significantly decreased (P<0·05) to about 60% of control levels. In 24-h cultures, the presence of OE2 and/or PHA had no effect. When separated T cell preparations from the tonsils were used, no significant effects of OE2 were seen in either the 10-h or 24-h cultures. In conclusion, OE2 downregulates the ER mRNA content in a tonsillar mononuclear cell system in vitro as it does in many primary oestrogen target cells.