The antibacterial effect of SPAD, a root canal obturation paste containing paraformaldehyde, was examined and compared with that of calcium hydroxide apexification paste (Pulpdent). Both materials were either immediately placed in cultures containing streptococci group D, isolated from infected root canals, stored in an incubator for 3, 7 and 15 days before their testing, Evaluation of the antibacterial effect was carried out by measuring the diameter of bacterial inhibition zone after incubation of the materials in cultures, for 24, 48 and 27 hours. Wide inhibition zones formed around SPAD, denoting it's strong antibacterial effect, were recorded. This effect decreased with time as a result of the decrease in the amount of formaldehyde released from the hardened material into culture. On the contrary, calcium hydroxide was unable to cause inhibition of bacterial growth throughout the period of the study.