The effects of 50 Hz sinusoidal electric currents and magnetic fields on the Gram-positive skin bacterium Propionibacterium acnes were investigated. Intracellular free calcium ([Ca 2 ] i ), intracellular pH (pH i ), and cell viability were examined, based on their relevance to ELF field studies and on previous studies conducted on P. acnes (UVA irradiation, photosensitization using porphyrin-based sensitizers, and broad-band red light). The [Ca 2 ] i and the pH i were measured spectrofluorimetrically using the fluorescent probes fura-2 and BCECF, respectively. Sham-exposed controls were used to assess the field exposed samples. Cell suspensions were exposed to 50 Hz, 0.2 mT sinusoidal magnetic fields generated by using Helmholtz coils for up to 30 min. The estimated maximum induced electric field was 0.2 mV/m. Changes in [Ca 2 ] i and cell viability were not detected. Ag/AgCl electrodes were used to expose cell suspensions to 50 Hz sinusoidal electric currents. The current densities were in the range 0.015±1500 A/m 2 (corresponding electric fields 0.01±1000 V/m). Changes in [Ca 2 ] i were not observed after current exposure. Current densities of 800 A/m 2 (electric field E 550 V/m) were required for a 50% reduction in cell viability. Current densities greater than 800 A/m 2 were required for a reduction in pH i . However, a pH gradient across the cell membrane (inside alkaline) was maintained even when exposure resulted in less than 0.2% survival (1400 A/m 2 , E 950 V/m). Thus, dissipation of the pH gradient across the cell membrane and changes in [Ca 2 ] i were not a consequence of cell inactivation by 50 Hz electric currents. This is in contrast to inactivation of P. acnes by UVA irradiation or photosensitization, where such changes have been obtained. Bioelectromagnetics 21:302±311, 2000.