To the Editor.— In the April 4 issue ofJAMA, Conant et al1indicate that commercial condoms, including those derived from sheep intestinal membranes (natural condoms), are effective in preventing the in vitro transmission of retrovirus infection.1Their conclusion is based on experiments in which condoms containing a solution of either mouse retrovirus or acquired immunodeficiency syndrome-associated retrovirus were fit over the plunger of a 12-mL plastic syringe, reinserted inside syringe barrels, and submerged in a beaker containing sterile culture media. After repeated aspirations and expulsions the authors found the beaker culture media to be free of retrovirus. In a somewhat similar series of experiments addressing the issue of hepatitis B virus infection, we reported that, while synthetic condoms do indeed prevent leakage of iodine 125-labeled hepatitis B surface antigen (HBsAg), natural condoms do not.2 There are a number of possible explanations for the apparent discrepancy between