During the growth of Isochrysis galbana, several non-protein amines may be detected in the growth medium. Of these, one (termed `TTl¿) accumulates in proportion to the numbers of cells present. The concentrations of `TTl¿, and of another (termed `TA¿), are 3¿5 times higher in cultures in which Isochrysis is predated by Oxyrrhis marina. The lowest estimates of the concentration of extracellular `TTl¿ are an order of magnitude higher than those of any protein amino acid. Of the protein amino acids, some like glycine are utilized during predation while others, like histidine, accumulate in the medium Because of the unknown N-content and reactivity of the non-protein amines during HPLC, it is not possible to say if these compounds (together with other components of dissolved organic N) form a significant proportion of the unaccounted for N in the system after predatory activity. During predation in the absence of detectable free ammonium (when Isochrysis may be expected to be N-deprived), particles accumulate in the medium. Most of these are <2.5 ¿.m in diameter and are suggested to be remains of digested prey. There is evidence of a reassimilation of these particles by prey-deplete Oxyrrhis.