In the course of the study "Toxoplasmosis and Prematurity" 330 blood samples from twins were examined. Our findings in a series of 21 premature twins (maternal sera were also examined) are reported in this paper. Toxoplasma antibodies were detected by the Sabin-Feldman test and specific IgM antibodies by the Remington test. The classical form of congenital toxoplasmosis was present in five pairs of twins, while toxoplasmosis was subclinical at birth in both twins of three pairs. The pattern of disease varied very much in seven pairs of twins. In one twin of two pairs signs of disease were present, while his cotwin appeared unaffected but with strongly positive result of SFT. The most interesting observation, however, is that in three pairs, one twin was infected and had evident congenital toxoplasmosis, while his cotwin was not, as proven by the disappearance of the Toxoplasma antibodies. This finding undoubtedly indicates the importance of whether the placenta is intact or not for the transmission of the infection.