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Department of Biological Sciences, University of Aston in Birmingham, Gosta Green, Birmingham, B4 7ET

(Received 17 April 1975)

Within a population of rat thymic lymphocytes there exists a group of cells which may be induced to enter prematurely the DNA synthetic phase and subsequently to enter mitosis (Whitfield, Rixon, MacManus & Balk, 1972). This response can be elicited in vivo by injections of calcium or magnesium chloride (Smith, Gurson, Riddell & Perris, 1975). Similarly, in isolated suspension cultures of thymic lymphocytes an increase in the calcium or magnesium concentration also evokes this response (Morgan & Perris, 1974).

The mitogenic action of calcium is not evident in female rats, which can be attributed to the presence of oestrogen (Smith et al. 1975). This can be simulated in the culture system by addition of 0·1 µg oestradiol/ml to the cell suspension (Morgan & Perris, 1974). Under these conditions the ability of raised