The activity of 4-oxoretinoic acid as an inducer of gap junctional communication was investigated in C3H/10T1/2 murine fibroblasts. Two isomers of this retinoid, all-trans- and 13-cis-4-oxoretinoic acid, enhance gap junctional communication. This is accompanied by increased expression of connexin43 mRNA. Decomposition fractions of canthaxanthin were isolated by preparative high-performance liquid chromatography and shown to be active in the cell-cell communication assay. Two of the decomposition compounds were identified as all-trans- and 13-cis-4-oxoretinoic acid. Therefore, it is concluded that the biological activity of canthaxanthin regarding cell-cell communication is at least in part due to the formation of active decomposition products such as 4-oxoretinoic acid.