The data presented by Gavis et al. (1981, J. Mar. Res., 39, 315–333) for the specific growth rate of phytoplankton in nutrient replete medium as a function of cupric ion activity can be described quantitatively by equations that relate the equilibrium binding of cupric ions to a cell receptor site. Statistically significant conditional binding constants could be estimated from the data for at least eight clones among three species. One clone could bind only one cupric ion per site. The others could bind a maximum of two cupric ions per site. For one of these clones K1 > K2. For the remaining six K2 > K1. For three clones K2 was so much larger than K1, that only their product β2 could be determined. Values of K1 ranged between 8.6 × 108 and 5.7 x 1010, values of K2 ranged between 3.1 x 108 and 1.9 × 1010, while those of β2 ranged between 3.5 × 1018 and 3.8 × 1020.