When uniform histologic criteria are applied to staging schistosome egg and granuloma development in the hamster liver, the evolution of the egg foci is shown to be monophasic, albeit with considerable variation of the individual cell response. Both real and artifactual egg-granuloma asynchrony are demonstrable. Alternate granuloma stages occur simultaneously within the same single organ, so that necrosis or fibrous scarring may result in some lesions but not in others. The granulomas of Schistosoma japonicum, S mansoni and S haematobium show both shared and distinctive features. Thus, oviposition is serial in S mansoni but clustered in the other two species. Neutrophils are common in S japonicum granulomas but are rare in the others. The differential features, listed in detail, will usually permit histologic identification of species during the early stages of infection; subsequently, the species-specific features and the overall intensity of host reaction tend to decline. At comparable egg loads and time spans, the liver pathology of S japonicum is the most severe. This is not related to granuloma size, but rather to more exudation and necrosis in early S japonicum granulomas, their tendency to encroach on adjacent liver tissue and to more extensive diffuse inflammatory infiltration. Hoeppli phenomena occur around S japonicum eggs both in stellate form, and as intraovular "reverse" precipitates. Plasma cells and amyloid deposition are frequent. Conversely, S haematobium lesions are less destructive than those of S mansoni. These findings can be correlated, to some extent, with current knowledge of the biology of schistosomes and of the antigenic components of their eggs, but several key problems concerning the immunologic host response remain to be solved.