Sulfo-N-succinimidyl derivatives of the long-chain fatty acids, oleic and myristic, were synthesized and covalently reacted with isolated rat adipocytes. The plasma membrane proteins labeled by these compounds and the effect of labeling on the transport of long-chain fatty acids were investigated. Sulfo-N-succinimidyl oleate (SSO) and myristate (SSM) inhibited the transport of fatty acids (by about 70%). Inhibition of fatty acid transport was not a result of alterations in cell integrity, as intracellular water volume was not changed. It did not reflect effects on fatty acid metabolism, since it was observed under conditions where greater than 90% of the fatty acid taken up was recovered in the free form. The inhibitory effect was specific to the fatty acid transport system, as the transport of glucose and the permeation of retinoic acid, a substance with structural similarities to long-chain fatty acids, were unaffected. Sulfosuccinimidyl oleate reacted exclusively with a plasma membrane protein with an apparent size of 85 kDa while sulfosuccinimidyl myristate also labeled a 75-kDa protein. These proteins were among the ones labeled by diisothiocyanodisulfonic acid (DIDS) which also inhibits fatty acid transport irreversibly. The data suggest that the 85-kDa protein, which is the only one labeled by all three inhibitors is involved in facilitating membrane permeation of long-chain fatty acids.