The utility of in vitro uptake of14C-glycine into fish scales as an index of immediate past fish growth was validated for white suckers (Catostomus commersoni), carp (Cyprinus carpio), yellow perch (Perca flavescens), and bluegills (Lepomis macrochirus). Good correlations were found for all species between growth over 2 or 3 weeks and the amount of glycine taken up by scales; starvation of fish reduced the uptake considerably. Although glycine uptake was dependent on growth rate of the fish and not simply on temperature at which scales were incubated in glycine, the relationship of glycine uptake to incubation temperature was not constant for fish acclimated to different temperatures. Thus, growth comparisons could not be standardized to a particular temperature by a linear regression. In field applications, scales should be incubated at, and glycine uptake reported for, ambient temperatures.