44,081 R.P., or 2-[(1,5,10,10a-tetrahydro-3H-thiazolo[3,4-b]isoquinolin- 3-ylidine)amino]-4-thiazole acetic acid, is a compound which selectively inhibits rhinovirus in cell cultures. The compound, which was unable to inactivate infectivity of virions, appeared to act prior to RNA and protein synthesis without affecting adsorption or penetration of virus in MRC5 cells. In contrast, it protected intracellular [5-3H]uridine-labeled virions against the effects of RNase treatment, indicating that inhibition of virus uncoating is the mode of action of 44 081 R.P.