Determination of antibody to toxoplasmosis, on the basis of DT, CF, and ST, has shown that antibody quantities may vary, in the course of pregnancy, but are characterised by defined behaviour.--Antibody levels are shown to be at their maximum in early gravidity, that is before the 18th week of pregnancy, followed by relative decline and widely constant values beyond the 25th week.--The author, in a comprehensive discussion, points at the similarity between curves and titres of HCG excretion in urine, and he consideres that similarity a possible approach to further studies in greater detail.--Reference is also made to the importance of the above findings to diagnosis of acute infection by Toxoplasma gondii.