The acute and subacute toxicity of 4-(p-chlorophenylthio) butanol (W-2719), on anti-allergy agent, was investigated in mice, rats and dogs. Acute LD50 values in the mouse (1145.0 mg/kg p.o.) and rat (greater than 1400.0 mg/kg p.o.) and maximum tolerated dose in the dog (420.0 mg/kg p.o.) were very high, indicative of a high degree of safety following a single oral dose. The subacute toxicity studies were conducted by repeated daily oral administration of the compound for 30 days. In the rat, W-2719 did not produce any significant toxicity up to a dose level of 100.0 mg/kg/day, when administered as a drug-diet admixture. A higher dose, i.e., 200.0 mg/kg/day, produced marked reductions in body weight gain, food consumption, RBC and WBC (females especially), and other hematological parameters. In the purebred beagle dog, W-2719 did not produce any significant toxicity up to a dose level of 100.0 mg/kg/day, the highest dose level tested in this species.