Minute amounts of trace elements can affect phytoplankton metabolic processes. Consequently, 14C primary productivity measurements may be adversely affected by contaminant metals inadvertently added during sampling and handling procedures. We analyzed various components used in the standard 14C method for metals; significant amounts of contaminants were found in many of the stock solutions used in this procedure and in associated containers. 14C uptake rates were lower under contaminated conditions than those with clean techniques. The latter are described in detail; their adoption is urged for more accurate and precise estimates of this fundamentally important process.