The in vitro uptake of gamma-emitting radionuclides by microfilariae of Dirofilaria immitis was investigated. Radionuclides tested were 133Ba, 207Bi, 82Br, 109Cd, 51Cr, 60Co, 59Fe, 203Hg, 125I, 54Mn, 32P, 125Sb, 75Se, and 65Zn. Only 207Bi, 59Fe, 203Hg, 54Mn, and 46Sc showed more than 2% of the available radioactivity to bind to the microfilariae. When tested for retention in vitro only 203Hg showed retention levels approaching 90%. Moreover, when dimethyl suiphoxide was incorporated into the medium at levels of 1% (v/v) the uptake of 203Hg could be increased by 3–5 times; no other radio-isotope tested responded in this manner. The uptake of 203Hg was directly related to temperature and time of incubation. Mercury, as mercuric chloride, was toxic to the microfilariae and respresents an impediment to the incorporation of high levels of 203Hg in microfilariae.