In secondary inner ear responses against keyhole limpet hemocyanin (KLH), the characterization of interleukin-2 levels in perilymph and serum developing over 5 days in Hartley guinea pigs was investigated using an immunoassay. No interleukin-2 levels were detectable at Day 0 in serum and perilymph. The earliest perilymph IL-2 levels were observed after 6 h, peaking at 18 h and decreasing to zero by Day 5. In contrast, during this observation period, no IL-2 was detectable in serum or perilymph from control ears. Since previous work has identified T-helper cells in the endolymphatic sac, this site appears to be the probable source of interleukins. This study provides further support for the existence of an inner ear immune response which is regulated by lymphokines. Furthermore, interleukins may be a chemoattractant for cells entering the cochlea during the immune response.