Target size analysis by radiation inactivation is widely used for molecular weight determination of membrane enzymes and receptors in situ without the need for prior solubilization or purification. However, since most molecular weight data available in the literature on membrane proteins involve the use of detergents for solubilization, the target sizes of membrane proteins in situ and after solubilization by detergent treatment have been compared. Using data from the literature and personal results, three different types of behavior of membrane proteins in presence of detergents were found: (i) uncoupling of subunits (electric eel acetylcholinesterase, placental steroid sulfatase, and human nonspecific beta-glucosidase); (ii) coupling of protein molecules (mouse liver neuraminidase, and rat liver insulin receptor regulatory component); and (iii) no major change in quaternary structure (rat liver insulin receptor, kidney gamma-glutamyltransferase, asialoglycoprotein receptor, insulin degrading enzyme, and human leucocyte neuraminidase). For all these proteins, there is a statistically significant increase in target size of about 24% over the value obtained in situ without detergent. A relatively large body of literature data involving a variety of membrane proteins, membrane types, and irradiation conditions (electron accelerators or 60Co sources, and proteins irradiated in lyophilized form or frozen solution) was examined, and it was concluded that target sizes of membrane proteins, irradiated in the presence of Triton X-100, should be diminished by a factor of about 24% to obtain the molecular weight value.