An immunological technique for detecting Chlamydia trachomatis and Chlamydia psittaci inclusions in infected McCoy cell cultures was developed by using a genus-specific monoclonal antibody to Chlamydia spp., rabbit anti-mouse immunoglobulin G bridging antibody, alkaline phosphatase-anti-alkaline phosphatase (APAAP) monoclonal antibody conjugate, and naphthol AS-phosphate/fast red substrate. Chlamydial inclusions stained red and were easily detected against a background of blue hematoxylin-stained nuclei. After 18 h, inclusions of C. trachomatis serovar L2 LGV434/Bu and C. psittaci strain 6BC were stained by APAAP but not by iodine or Giemsa. At 48 h inclusion counts were significantly higher in the APAAP cultures. Both the APAAP procedure and conventional staining detected 35 of 239 (15%) cultures 48 h after inoculation with urethral or endocervical specimens. However, at 24 h after inoculation 22 of 35 (63%) were positive by APAAP staining while negative by iodine. This immunostain also allowed identification of chlamydial inclusions in endometrial biopsies from patients with tubal factor infertility or pelvic inflammatory disease.