169 obtained with a C/L ratio of about 1/0 * 6 and a Ch/L ratio of about 30/1. The experiments were carried out with a synthetic, saturated lecithin, crystalline L-a-di-myristoyl phosphoryl choline (Baer, 1953) . The same series of experiments showed that the storage for 4 months at 56°C. of antigens prepared with the crystalline synthetic lecithin in double the usual concentration resulted in a greater loss of sensitivity than the storage of antigens containing the usual lecithin

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... This observation suggested that the lecithin was significant for the keeping qualities of cardiolipin antigen. The purpose of the present investigations was to study the influence of variation in the concentration of the natural lecithins (egg and beef-heart) and to test whether the response to variation was uniform for syphilitic and biologically false-positive sera. In addition, it was decided to te3t whether the observation on the correlation between the lecithin concentration and the keeping qualities could be applied to the natural lecithins. Recently, Tonks, Allen, and Fowler (1955 a, b) reported that different amounts of various lecithins were needed to obtain "standard sensitivity", and Kline (1950 Kline ( , 1954 Kline ( , 1955 found that the C/L ratio in his test should be altered from about 1/10 to about 10/9 when crystalline synthetic L-a-di-myristoyl lecithin was used instead of natural lecithin.t It was previously found (Reyn and others, 1956) that the average sensitivity obtained was the same as that of a reference antigen prepared with egg lecithin (International Reference Preparations, 1953) when the concentration (molecular weight basis) of the crystalline synthetic lecithin was reduced to about i of that used for the egg lecithin in the reference antigen. Previous observations (Reyn and others, 1956), as well as preliminary experiments in the present series, t The preparations were compared on a unit/weight basis.