In laboratory-scale filtration experiments conducted with settled water from a lime-softening plant, breakthrough was discovered to be greatly influenced by particle detachment. Detachment was studied explicitly through a carefully designed experimental procedure that incorporated a sudden decrease in the influent particle concentration at the end of lengthy filter runs. Particle detachment (and, consequently, breakthrough) occurred most dramatically in an intermediate size range for the suspension studied; this intermediate size range coincides with that commonly associated with Cryptosporidium oocysts. Includes 20 references, tables, figures.