In the preceding paper ( 1), there was presented a summary of the maximum titers of cold agglutinins found in 1069 cases of a variety of conditions, including the various infections of the respiratory tract that are commonly encountered in this vicinity. Significant titers (in dilutions greater than 20) were Methods. The method used in the tests for cold agglutins was given in detail in the preceding paper (1). For the present study, the specimens of venous blood were all collected under sterile precautions and the serum was separated from the clotted blood, either at room tempera-1 Hereafter referred to for brevity as "atypical pneumonia." ture or, more often, after the blood had been kept at 370 C. for a short while. The cleared sera were stored in rubber stoppered tubes at 5 to 100 C. Specimens, averaging 3 per patient, were obtained at suitable intervals. The tests were carried out either on the day when the blood was obtained or within a few days. Equal volumes of serum and 2 per cent saline suspensions of 2-to 4-day-old red blood cells from individual group 0 donors were used. The titers are recorded as the reciprocals of the highest final dilutions of serum giving 1 + agglutination at 0 to 50 C. and complete dispersion at 370 C. RESULTSThe conditions under which the present cases were studied unfortunately did not permit frequent observations throughout the entire course of the acute illness and prolonged observations during convalescence in every instance. Data concerning certain of the features to be considered are, therefore, based on limited numbers of cases. In the analysis which follows, the time relationships are referred to the day of onset of symptoms. In most of the cases, this could be ascertained quite accurately, but, in a few instances, it was only approximated. Time of appearance, decline, and disappearanceThe first postitive tests ( Figure 1A) were observed between the seventh and twenty-seventh days, but mostly during the second or third week. The last negative tests in the same cases were obtained before the end of the second week in all but 2 of them. The maximum titers (Figure 2) occurred mostly between the eleventh and twentyfourth days. There were a number of cases, however, in which the maximum titers were not attained until the fifth week or later, and in 4 cases, maximum titers of 40 to 160 were observed between the fourth and eighth days and lower titers obtained later. There was no definite correlation between the maximum titers and the time 458

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