An evaluation was made of an interdigital skin test to assess cutaneous basophil hypersensitivity (CBH), mediated by thymic (T) cells, in chickens less than 2 wk of age. Hubbard-by-Hubbard roosters 3, 5, 7, 10 and 14 days of age were injected intradermally in the interdigital skin between Digits 3 and 4 of the right foot with 100 micrograms or 200 micrograms of phytohemagglutinin-P (PHA-P) in .10 mL of physiological saline solution (PSS). The left foot was injected with PSS and served as a control. The CBH response elicited by the PHA-P was evaluated by determining interdigital skin thickness before injection and at 12 and 24 h after injection. The effect of treatment with the T-cell suppressing drugs dexamethasone and cyclosporin on the CBH response was determined in chickens treated daily for 4 or 5 days prior to skin testing at 5 or 10 days of age. Significant increases (P less than .005) of .64 to .88 mm occurred in the mean, interdigital skin thickness for all age groups of chickens at 12 and 24 h after injection of 100 or 200 micrograms of PHA-P. The PHA-P-elicited. CBH response was significantly suppressed by treatment with dexamethasone (P less than .005) or cyclosporin (P less than .05). The results indicated that the interdigital skin test is a rapid and simple means of assessing normal and suppressed immune responses in chickens at 3 to 14 days of age.