Non-sticky latex beads and sticky diatoms were used as models to describe mutual coagulation between sticky and non-sticky particles. in mixed suspensions of beads and Thalassiosira nordenskjoeldii, both types of particles coagulated into mixed aggregates at specific rates, from which the interspecific coagulation efficiency could be calculated. Stickiness between beads and T. nordenskjoeldii was 50% of that of T. nordenskjoeldii in monospecific suspensions, and this ratio remained constant throughout 12 experiments covering 1 order of magnitude variation in the stickiness level of T. nordenskjoeldii. Mutual coagulation between Skeletonema costatum and the non-sticky cel:ls of Ditylum brightwellii also proceeded with hall the efficiency of S. costatum alone. The latex beads were suitable to be used as 'standard particles' to quantify the ability of phytoplankton to prime aggregation of suspended particles.