SUMMARYConditioned seawater, removed by filtration from an exponentially growing mixed phytoplankton population, inhibited the growth of a small inoculum of cells from the same source without reducing the lag period. When the 2 populations, one in exponential growth and the other freshly inoculated, were separated by a filter membrane allowing passage of excreted compounds, the growth rate of the freshly inoculated cells was again depressed but the lag phase was reduced almost as effectively as by the addition of the chelator, EDTA. Thus both inhibitory and stimulatory compounds appear to be excreted by the cells during exponential growth, the stimulatory group, apparently involved with trace metal metabolism, being more labile to degradation.A test was made to determine if the population could be induced to utilize unconditioned water. Even after repeated transfers there was no reduction in the lag period, indicating that the initiation of exponential growth represents a response to excreted conditioning agents (possibly chelating compounds) rather than an adjustment by the population to use unconditioned water.