After a brief introduction, we shall present studies on the reproducibility of the techniques, a consideration of the sensitivity of the method and, finally, a n application that is presently under consideration. I n 1948 a research program studying the immunizing properties of fractions of the tubercle bacillus was in progress in the Bacteriology Department of the University of Michigan. In this study, complex extracts were prepared from mass cultures of the organism and were tested in guinea pigs and mice for their capacity to cause alterations in the normal resistance of the animals to tuberculosis. About this time, one of us (HMR) approached the Bacteriology Department with an inquiry as to whether there were any lines of research in which infrared spectroscopic studies could make a contribution to the study of complex biological mixtures. The feasibility of such an investigation in the work on immunity in tuberculosis was evident. The fact that the extracts being prepared from the tubercle bacillus were of a lipide nature proved to be an important factor in the progress made in the present research. Studies o n Reproducibility After several abortive attempts to find a correlation between the immunizing potency of a given preparation and its spectrum, we decided that it would first be necessary to study the system with which we were working to determine the degree of reproducibility of bacterial extracts prepared under similar conditions and the extent to which the different variable factors could be altered without affecting the extracts. Working with total CHCls-soluble fractions and subfractions prepared by precipitation with acetone, we were able to show' that, with sufficient care, different lots of tubercle bacilli grown and extracted under similar conditions yielded substances, the infrared spectra of which were sufficiently reproducible to warrant further study. Following this, a study was made of the effects of age of the inoculum, age of culture, temperature of incubation, composition of the medium, and duration of extraction on the reproducibility of the infrared spectra of the lipide extracts. I t was determined that all of these factors were critical and that, if varied, they would lead to changes in the composition of the lipide mixtures.

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