In active cases of ocular toxoplasmosis, antibodies against the parasite are produced locally in the eye. This antibody production may be revealed by examination of the aqueous humour, and comparing the antibody titer with that of serum. We examined the dye test titer as well as the total IgG concentration of the aqueous humour and serum in 10 patients with active retinochoroiditis. Six of the 10 patients had a significantly higher antibody titer in the aqueous humour than in serum, in relation to their contents of total IgG, proving that the retinochoroiditis was caused by infection with Toxoplasma gondii. Negative results do not exclude the possibility that the inflammation is caused by this parasite. False negative results may occur when the total IgG concentration is either very low, or very high. The time at which the test is performed, in relation to the onset of the inflammation, is also of importance. Our results also suggest that the larger the size of the active lesions is, the higher is the chance of obtaining a definite diagnosis. It is concluded that aqueous humour analysis for antibodies against Toxoplasma gondii should be considered in cases of active retinochoroiditis of unclear or suspected toxoplasmic etiology.