This study has demonstrated the extreme sensitivity of Chlamydia trachomatis growing in McCoy cells to small changes in external amino acid supply. In the absence of cycloheximide, a decrease in the amino acid concentration of medium to 75% of control values was sufficient to induce the growth of enlarged chlamydial forms of reduced infectivity. Morphology became more distorted and the yield of infectious particles from inclusions declined as medium amino acid levels were further reduced. These events correlated with a general decline in intracellular amino acids, as measured by high-performance liquid chromatography, suggesting that chlamydiae require a minimum concentration of each amino acid for normal development. Cycloheximide enhanced the production of normal organisms and increased infectivity yield in media, suggesting that the drug increased the available pool of amino acids. This was supported by intracellular amino acid analyses. Aberrant forms with reduced infectivity were also induced during supply of infected cell cultures with medium containing blood plasma amino acid concentrations, supporting the proposal that nutrient levels in vivo could promote abnormal chlamydial development. Markedly abnormal forms were also observed during glucose deprivation, providing further evidence that aberrant development is a general stress-related response.