Ordinary biological methods of evaluating protein quality are subjected to various sources of error, such as preliminary feeding, body weight of the animals, length of feeding period, endogenous nitrogen metabolism, etc. (1). Wide variations in the reported data may be partly due to this fact. The author postulated that by applying labelled protein these shortcomings may be avoided and that a balance study using nitrogen-15 would give most reliable data, uninfluenced by other factors which are unavoidable in ordinary balance studies using unlabelled nitrogen (2). However, the most important prerequisite for testing this assumption is to obtain labelled protein. As it has been known long since that chlorella contains a large amount of protein and can be easily cultured in quantity (3, 4), it was thought that labelled protein might be obtained by adding $N^{15}$ into the culture media. In this study it has been tried to find out, therefore, whether it is possible to obtain labelled protein by incorporating $KN^{15};O_3$ into the culture fuid and at what stage of growth the incorporation of the label into chlorella occurs.