Adherent adult and cord blood macrophages were infected with human coronavirus OC43 at a multiplicity of 1-1.5 and washed twice to remove unbound virus. Virus progeny was detected in the supernatant on day 1 and peaked at 2-3 days at an average titer of 5 +/- 3.9 x 10(6) pfu/ml from seven samples. Viral RNA was detected by nested set RT-PCR in infected macrophages incubated for 48 hr. Intracellular viral nucleocapsid was detected in 15% of the cells and surface staining for viral spike antigen was observed using monoclonal antibodies. Amplification of infectious virus and detection viral RNA and antigen synthesis in macrophages in vitro indicates susceptibility to OC43 virus.