An in vitro test for the differentiation of trachoma strain isolates was described. The method was based on absorption of trachoma antiserum followed by immunofluorescent staining of individual trachoma strains.

Nineteen trachoma strains were studied; two main types were identified. Evidence for quantitative and qualitative antigenic variation between the two types was presented. The limitations in practice and in theory of the test were emphasized.

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