The induction of phenol-sulfate conjugating activity was studied by exposure to some phenolic compounds and also the duration of its activity induced by PCP (I) in the midgut gland of short-necked clam Ruditapes philippinarum was studied.The inductivity of phenol-sulfate conjugating activity in short-necked clam among PCP, resorcinol, p-cresol, p-chlorophenol, p-nitrophenol, and phenol was compared; PCP was the most effective inducer, resulting in a 2.4-fold activity in the control clam during the 3-day exposure period to 0.1 ppm PCP.A subsequent experiment was performed to elucidate the effect of long-term exposure to sublethal PCP (0.1 ppm) for 5 wk on the phenol-sulfate conjugating activity in clams, feeding on a unicellular green alga Dunaliella species by dipping the clams in its growing seawater-medium (∼4 × 104 cells/mL) for 2-3 h every other day, when the test PCP-water was replaced by a newly prepared one.The conjugating activity in the clam increased with the time of exposure to PCP and reached 42 nmol/min/g-tissue at the 5-wk period, corresponding to ∼7-fold with that in the control.When the clams which were exposed to 0.1 ppm PCP for 2 wk and then transferred to PCP-free seawater, its induced activity of 24 nmol/min/g-tissue decreased gradually, but it was still at a level of 1.5-fold with that in the control even after 3 wk, although the biol. half-life of PCP accumulated in the clam was only ∼1 h.