An experimental system based on an improved version of an existing α-particle irradiator has been developed for radiobiological studies, in particular those investigating bystander effects. It consists of a 20-mm-diameter stainless steel chamber that can be equipped alternatively with ²⁴⁴Cm or ²⁴¹Am sources of different activities. Mylar®-based petri dishes 56 mm in diameter were specially designed to house adaptors for permeable membrane inserts that reproduce the geometry of commercial cell culture insert companion plates. Characterization of the radiation field at the cell level was performed by experimental measurements and calculations. The average incident LET was about 122 keV/μm for ²⁴⁴Cm and about 125 keV/μm for ²⁴¹Am. Dose rates at the chosen source-sample distance were 2.8 and 88.6 mGy/min, respectively. These low dose rates are suitable for our planned experiments on low-dose effects. For both sources, the uniformity of the α-particle dose was better than ±7%, and the photon dose calculated at the cell entrance was negligible compared to the α-particle dose. The irradiator is small enough to be inserted into a cell incubator for irradiation under physiological conditions or into a refrigerator to prevent metabolic processes during irradiation. Benchmark experiments using the ²⁴¹Am source to examine DNA double-strand breaks in directly hit and bystander primary human fibroblasts have shown that the irradiator can be used successfully for bystander effect studies.