Cd²⁺ uptake rates in the cyanobacterium Anacystis nidulans IU 625 (ATCC 27144) were found to be concentration-dependent. While Ca²⁺ and Zn²⁺ showed competitive inhibition, Hg²⁺ enhanced Cd²⁺ uptake. The pH values optimal for growth of the organism (pH 8.5) resulted in more Cd²⁺ uptake than low (pH 6.5) or high pH (10.0) conditions. Dithiol (dithiothreitol) proved more effective in lessening Cd²⁺ uptake than monothiol (mercaptoethanol). The test organism suspended in spent medium showed negligible Cd²⁺ uptake. Among the sythetic complexans, EDTA inhibited metal uptake, while citrate favored the process. There was more Cd²⁺ uptake by dense cultures, but the specific rate expressed per unit protein was less.