As the most prominent acyclic derivative of guanine, 9-[(2-hydroxyethyl)methyl]guanine (acyclovir, ACV) is now in clinical use for the treatment of diseases caused by herpes simplex viruses (HSV). However, ACV-resistant mutants of HSV are increasing as a result of loss or alteration of thymidine kinase (TK) activity or alteration in the virus DNA polymerase. We started the screening in order to search for novel antiherpetic agent in place of ACV. Recently, we isolated a new antiherpetic agent, AH-135Y, from the culture filtrate of a streptomycete, Streptomyces albovinaceus strain No. AH-135. This compound turned out to belong to the glutarimide antibiotics and has antiviral activity against herpes simplex virus (HSV)-I. Among 520 Streptomyces strains tested, a potent strain for the antiherpes agent, strain No. AH-135, was selected. This strain was isolated from a soil sample obtained from Yaku-shima, Kagoshima Prefecture and classified as Streptomyces albovinaceus No. AH-135. AH-135Y was purified from the culture broth via centrifugation, ethyl acetate extraction, and column chromatography on Diaion HP-10 and HP-20SS, with a yield of 10 mg. Its physico-chemical properties were determined, and structure was elucidated using ¹H NMR, ¹³C NMR, ¹H-¹H COSY, ¹³C-¹H COSY, and HMBC data, revealing a structure similar to actiphenol with a methyl group at C-3 instead of a hydroxymethyl group. AH-135Y showed antiherpetic activity with an EC₅₀ of 2.4 μg/ml against HSV-I and cytotoxicity against Vero cells with an IC₅₀ of 30.0 μg/ml, resulting in a selectivity ratio (IC₅₀/EC₅₀) of 14.3. It also exhibited antifungal activity exclusively against Saccharomyces cerevisiae (MIC = 1 μg/ml) but no antibacterial activity. Though AH-135Y contains an aromatic ring in its structure, it demonstrated strong activity against HSV-I. Details on the mechanism of action of AH-135Y remain to be examined.