Para-aminobenzoic acid (PABA) is an early interferon inductor. The present study assesses the interferon-inducing activity of PABA (0.007 and 0.06% solutions) and poludan (Poly A:U) injected subconjunctivally to rabbits. Interferon (If) titer in the conjunctival washings and anterior chamber humor was assessed 4-48 h after injection of If inductors. After the first injection of 0.007% PABA, If titer in the conjunctival washings was constantly increasing and reached the maximum after 48 h (64-128 U/ml). Repeated injection of PABA after 5 days still more stimulated the production of If, with the peak after 24 h (128 U/ml); after 48 h the titer of If was still high. If titers induced by 0.007% PABA and poludan were compatible after both injections in the conjunctiva but not in the anterior chamber humor. With poludan, the maximum If titer (16 U/ml) was observed 6 h postinjection, after which it was no longer detected, while after PABA the maximum If titer (64 U/ml) was observed during the 4th and 12th hours postinjection and then decreased, remaining rather high after 24 h. After 0.06% PABA, If titers were 2-4 times lower in all experiments than after 0.007% PABA. The detected interferon-inducing activity of PABA suggests its therapeutic efficacy in ocular diseases involving disorders in If production.