Cultured fibroblasts isolated from murine livers by tissue trypsinization were exposed to a static magnetic field (0.49 T) and an extremely low frequency (ELF) magnetic field (50 Hz, 0.020 T). The cultures were exposed to magnetic fields on four consecutive days for exposure times of 2, 4, 8, 16, 32 and 64 min. After exposure the following parameters of the fibroblast cultures were determined: the dynamics of culture growth, the protein content and C-14-thymidine incorporation. The cytometric parameters of the fibroblasts were also assessed. The ELF magnetic field compromised vital functions of the fibroblasts (inhibition of culture growth, decreased cellular protein, lowered cytometric parameters) and caused a slowdown in the rate of C-14-thymidine incorporation which indicates altered DNA synthesis. Ongoing experiments with a static magnetic field have shown no effect on the vital functions of fibroblasts.