The levels of methyl mercury in lake and river sediment from the St. Clair system have been investigated in relation to total-mercury concentrations and to levels of ethyl-lead compounds which were detected during the work. The co-occcurrence of ethyl-lead and methyl mercury indicate a connection probably in the form of a chemical methylation of mercury from methyl groups among the ethyl ones in alky-lead. In the incubated river and lake sediment from the St. Clair system no biological methylation of mercury could be statistiaclly certified.