Using a chemiluminescence (CL) test, it had been previously demonstrated that Vibrio pectenicida, which is pathogenic to Pecten maximus larvae, was able to inhibit completely the CL activity of P. maximus hemocytes and partially inhibit those of Crassostrea gigas. Conversely, a Vibrio sp. strain, S322, pathogenic to C. gigas larvae was more active in reducing the CL activity of oyster hemocytes than of scallop hemocytes. Using this same CL biotest, V. pectenicida and S322 cytoplasmic extracts were shown to reproduce CL inhibition while the cytoplasmic extract of a nonpathogenic strain (U1, Pseudoalteromonas) was without effect. Moreover, cytoplasmic extract as well as live V. pectenicida cells provoked, within a few hours, death of P. maximus hemocytes adhering to a glass slide. After partial purification, it was shown that toxic activities of V. pectenicida cytoplasmic extract was due to a toxin, named VHKT (for vibrio hemocyte- killer toxin), which is heat stable, acid and protease resistant, and less than 3 kDa in molecular weight. Attempts to purify VHKT by reverse-phase (C18) HPLC separated activity into the fraction eluted by water at a retention time of 4.02 min.