The effects of dissolving horseradish peroxidase (HRP) or HRP conjugated to wheatgerm agglutinin (WGA-HRP) in radiocontrast media (MD76) on the intraaxonal transport and enzymatic activity of these tracers were evaluated both in vivo and in vitro. The in vivo studies showed that more reaction product was visible in the L4 dorsal horn following soaking of the sciatic or peroneal nerve, or following cutaneous injection of WGA-HRP, when WGA-HRP was dissolved in MD76, as compared to dissolving WGA-HRP in distilled water. Additionally, there appeared to be an enhancement of anterograde transport and a reduction of retrograde transport of WGA-HRP when this tracer was dissolved in MD76, as compared to dissolving it in distilled water. The in vitro studies indicated that radiocontrast media increased the enzymatic activity of both WGA-HRP and HRP as compared to their enzymatic activity in distilled water when assayed spectrophotometrically. Data are presented that indicate some binding (about 4%) of HRP with MD76. This binding of HRP with organically bound iodine may account for the enhancement of anterograde transport and/or increased enzymatic activity.