1. Cross-immunity tests between strains of rural typhus and tsutsugamushi in the guinea-pig, rabbit and monkey were made. Complete cross-immunity between the strains was demonstrated.
2. The problem of the absence of a primary ulcer in rural typhus and its presence in tsutsugamushi is discussed. Experimental findings are recorded; from consideration of these and certain clinical and epidemiological observations, the conclusion is drawn that one and the same virus may cause gradations of dermal lesion that vary greatly in extent and duration.
3. Correlation of the results of cross-immunity tests and experimental infections with clinical, aetiological, epidemiological and serological findings indicates that the two diseases are identical. Rural typhus is not a disease sui generis, and the term should be discarded, the older designation, "tsutsugamushi disease ", being retained.