This study was conducted to determine the effect of modified crude protein (CP) concentrations and a supplemental directed microbial in the milk replacer fed to Holstein calves during elevated ambient temperatures. Fortyour Holstein calves were randomly assigned treatments in a 2x2 factorial arrangement of treatments. Weekly and daily measurements were taken and analyzed using PROC MIXED in SAS. Calves fed increased CP concentrations in the milk replacer had greater body weight and frame measurements during the pre-weaning phase and at weaning. Directed microbial supplementation led to increased ADG during the post-weaning phase. Directed microbial supplementation and milk replacer modification helped alleviate heat stress through decreased rectal temperatures and rectal scores. Directed microbial supplementation increased fecal scores. Calves fed the control diet had greater volatile fatty acid concentrations in the rumen fluid, showing signs of increased rumen papillae development.