Chlamydia trachomatis is normally grown in McCoy monolayer coverslip cultures using partially defined media containing foetal calf serum at concentrations up to 10% (v/v). Omission of the serum decreased the number of inclusions produced by infecting the McCoy cells with a standard inoculum of a genital strain of C. trachomatis. Substitution of the foetal calf serum with a macromolecular fraction from the serum or with a mixture of sodium oleate, bovine serum albumin fraction V and fetuin maintained the inclusion count, but substitution with serum filtrate, containing the amino acids and other low molecular weight components, decreased the inclusion count of C. trachomatis. The role of calf serum and the need for a fully defined medium excluding serum for studying the growth of C. trachomatis in tissue culture are discussed.