The author has previously shown that the addition of egg-white lysozyme to the reaction mixture used in the Treponema pallidum immobilization test (TPI test) can reduce the time required for immobilization from 18 hours to as little as 6 hours. This opened up the possibility of developing a one-day TPI test and of further simplifying the procedure, as the conditions do not need to be so strictly controlled to ensure survival of the treponemes for the shorter time. In this paper it is shown that the standard procedure of incubation under an atmosphere of nitrogen and carbon dioxide can be replaced by incubation under a layer of liquid paraffin. The survival of treponemes is satisfactory under these conditions and the oil technique does not alter the sensitivity of the 6-hour test with added lysozyme. Comparative tests using the standard procedure, the lysozyme-gas technique and the lysozyme-oil technique showed that both modifications give the same degree of sensitivity and specificity after incubation for 6 hours as does the standard method. The feasibility of introducing these modifications into routine laboratory practice is discussed.