Serum lithium estimation has recently assumed great importance in psychiatry. It can be successfully undertaken in any ordinary hospital laboratory equipped with a simple flame photometer such as the EEL (Evans Electroselenium Limited). It has been found that de-proteinization of serum is very satisfactorily achieved by addition of hydrochloric acid and subsequent heating. This avoids the retention of lithium in the precipitate which is associated with the use of trichloroacetic acid and of ethanol. The method is simple for the technician and very reliable and accurate.