The relative acute toxicity of particle-bound and dissolved interstitial cadmium was investigated using a new bioassay procedure. Interstitial concentration of Cd was controlled by means of peristaltic pumps, allowing separate manipulation of interstitial and particle properties. Addition of small quantities of organic-rich fine particles to sandy sediment resulted in greatly differing particle-bound Cd concentrations in sediment with similar interstitial Cd concentrations. Analysis of variance indicated no significant difference in the survival or ability to rebury in sediment of the phoxocephalid amphipod Rhepoxynius abronius (Barnard), when exposed to sediment with different total Cd concentrations but nearly equal interstitial Cd concentrations; in one case LC50 data indicated slightly increased mortality in sediment with higher total Cd concentration. At least 70.2-87.9% of mortality could be predicted from past data on mortality based on dissolved Cd concentrations. The acute toxicity of Cd to the infaunal amphipod appears to be due principally to Cd dissolved in interstitial water.