When freshly isolated rabbit lenses were co-cultured with iris-ciliary body (IC), the protein synthetic activity of these lenses decreased significantly. The inhibitory effect is temporal as longer incubation showed greater inhibition. it was also found that the presence of IC in the culture medium increases the sodium ion levels in the lenses and causes some decrease in the potassium ion levels. Both the protein synthetic activity and cation levels of the lenses cocultured with IC are at least partially reversible. It seems that the IC-derived factor(s) has a molecular weight of more than 1 kDa. It is not clear from these experiments whether the altered cation balance is responsible for the inhibition of the protein synthesis.