In an effort to highlight potential "cell division cycle proteins" in the fission yeast, Soh-izosaoaharorrtyces pombe, cultures growing exponentially in defined medium were starved of nitrogen. This treatment causes a semi-synchronous burst of accelerated cell division at a reduced cell size and can thus be envisaged as an uncoupling of 35 the growth cycle from the DNA-division cycle. S-methionine labelling during the period of accelerated division reveals the differential synthesis of two polypeptides of molecular weights 46kd and 27kd. In order to distinguish between the alternative hypotheses that these proteins ( denoted as p46 and p27 ) are a) "division proteins", or b) "stress proteins" (synthesized as a direct result of nitrogen starvation), use was made of the temperature-sensitive mutant of S.pombe, wee 1.50, which, at the restrictive temperature, divides at half the size of wild type cells. This mutant does NOT accelerate into division upon nitrogen starvation at the restrictive temperature for reasons satisfactorily explained by current models of cell cycle 35 control in S.pombe. S-methionine labelling during the period following nitrogen starvation does, however, reveal the same differential synthesis of proteins p46 and p27 as seen in wild type. Heat shock analysis reveals the existence of two heat shock proteins in S.pombe of similar sizes to p46 and p27. It was therefore postulated that nitrogen starvation in this yeast induces a subset of the total array of heat shock proteins. The hypothesis is supported by the observation that nitrogen-starved cells acquire thermotolerance at a time only shortly after the transient appearance of proteins p46 and p27. A heat shock protein in Saooharomyces cevevisiae , HSP 48, has recently been identified as the glycolytic enzyme ENOLASE. Furthermore, it has been proposed that this protein may be solely responsible for the acquisition of thermotolerance in pre-heated cells. Using an anti-HSP 48 antiserum the possibility that, in S.pombe, protein p46 is enolase was tested by immunoblotting techniques. While the antiserum reacted specifically to a single polypeptide of S.pombe of approximate molecular weight 50kd, the amount of this antibody-binding protein was not found to increase during nitrogen starvation. On this basis, it was concluded that protein p46 is not enolase. The implied divergence in the heat shock responses of budding and fission yeasts is emphasized further by the suggestion, by similar inmunoblotting techniques, that, in S.pombe, enolase is not a heat shock protein at all.