Among various nucleoside 5'-alkylphosphates, uridine 5'-hexadecylphosphate (UMPC16) and adenosine 5'-hexadecylphosphate (AMPC16) inhibited the sexual agglutination between a and alpha haploid cells of Saccharomyces cerevisiae. The effect of AMPC16 accompanied severe growth inhibition of the yeast cells but it was not observed with UMPC16. Sexual agglutination was not inhibited by the presence of UMPC16 or AMPC16 when the yeast cells had been pretreated with the mating pheromone. UMPC16 was characterized as a specific inhibitor of sexual agglutination without direct influence on the agglutinin function, being distinguishable from any of those ever known.