The responsiveness of chemoreceptor neurons depends on a combination of perireceptor and receptor events. Olfactory neurons of crustaceans are packaged into distinctive cuticular sensilla called aesthetascs. The cuticle of aesthetascs is thin and permeable, even though it does not contain any obvious surface pores or pore tubules. This suggests that this 'spongy' aesthetasc cuticle may act as a molecular sieve that restricts large odorant molecules from entering the sensilla and binding to the olfactory neurons. We examined whether this is so for the aesthetasc cuticle of the Caribbean spiny lobster Panulirus argus. We used a chromatographic column packed with aesthetasc cuticle and connected to a flow-through ultraviolet spectrophotometer to measure the elution times of ultraviolet-absorbent molecular mass markers between 165 and 2 x 10(6) Da. Molecules larger than approximately 8.5 kDa had similar elution times, indicating that they did not penetrate the cuticle. Molecules smaller than 8.5 kDa had longer elution times that were directly and inversely proportional to their molecular mass. These results suggest that aesthetasc cuticle excludes molecules larger than 8.5 kDa from having access to the olfactory receptor neurons. We conclude that the molecular sieving capacity of the aesthetasc cuticle of P. argus is a perireceptor mechanism that is a critical determinant of the types of molecules capable of stimulating its olfactory receptors.