We investigated the effects of polyoxyethylene sorbitan monolaurate (Tween 20) in the detection of IgG anticardiolipin antibodies (aCL) by the CL-beta 2-glycoprotein-I and the standard aCL solid-phase immunoassays. We found that Tween 20 disengages cardiolipin from a variety of microtiter wells rendering aCL undetectable by both methods. Our results agree with a previous report but are discordant with others. We offer rationale that may explain some of the discrepancies. Based in our findings, we do not recommend the use of Tween 20 for the detection of aCL.