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Male guinea-pigs were inoculated by the testicular route with a suspension of chlamydo-arthrospores of the filamentous phase ofC. immitis. The following histopathological changes were observed: voluminous pyocytic foci as well as granulomatous mononuclear reaction was initially observed and granuloma with multinucleated giant cells were seen 20 days after the inoculation. The following changes of the microscopic aspect of the parasite were correlatively registered: the so-called primary infection type of sporangia appear in great number three days after the inoculation. This type of sporangium is characterized by its great diameter up to 98 µ, peripheral endospore formation leaving a large central vacuole which frequently contain residual protoplasm. The endospores are thin walled, polyhedral and get free through an ostiole. Sporangia completely filled with globose endospores (the cystic type of sporangia) with radiate acidophilic formations on the peridial wall were observed in the testicles of the guinea-pigs killed 6 days after the inoculation. Reduction in number and in the size of the parasite were seen after the 20th. day of the inoculation. Radiate formation of the cell wall of the parasite appeared simultaneously with precipitin antibodies and would be the expression of antigen-antibody reaction.