The rapid development of biotechnological methods provides the potential of dissecting the molecular structure of microorganisms. In this review the molecular biology of chlamydia is described. The genus Chlamydia contains three species C. trachomatis, C. psittaci, and C. pneumonia which all are important human pathogens. Chlamydia is obligate intracellular bacteria with a unique biphasic life cycle. The extracellularly chlamydial elementary bodies (EB) are small, metabolic inactive, infectious particles with a tight outer cell membrane. After internalization into host cells the chlamydial structure changes, they transform to reticulated bodies (RB) which become larger, metabolically active, and start to replicate. Fourtysix hrs post infection RB reorganizes to EB followed by burst of the inclusion. The structure of the EB outer membrane differs from the membrane of gram-negative bacteria since it is highly cross-linked by S-S bridges. There are, however, also similarities to gram-negative cell walls. The chlamydial major outer membrane protein, Omp1, forms pores and is closely associated with lipopolysaccharide, LPS. LPS, however, is more loosely associated with Omp1 than in other gram negative bacteria since incubation of EB with antibodies against LPS will liberate it from the chlamydial surface. Therefore the surface localized LPS may be important for chlamydial survival. OMP1 varies between the different serovar of C. trachomatis. Several very conserved regions are separated by variable domains. The variable domains are very antigenic and are localized at the surface of EB. After chlamydial internalization into the host cell transition to RB starts. Some of the early proteins are DnaK-like and groEL-like heat-shock proteins. The chlamydial DnaK-like protein is very antigenic. Patient serum samples will recognize the chlamydial DnaK-like protein. From the determined DNA sequence the amino acid sequence was determined. It was 57% homologous to the Eschrichia coli DnaK protein. Also the GroEL-like protein is antigenic and very conserved. Factors of importance for pathogenicity of chlamydia have not yet been found. The adhesin(s) is unknown, and no factor of importance for the inhibition of fusion between phagosome and host cell lysosomes has been described. A protein similar to the mip gene product of Legionella pneumofila may be a possible candidate for a pathogenicity factor. Diagnosis of C. trachomatis infections has been done by chlamydia cultivation in tissue culture cells, by immunofluorescence and by ELISA. A new method based on the polymerase chain reaction (PCR) has been developed. As primers sequences from the common plasmid were used. This method has high sensitivity and specificity and does not require live chlamydia.(ABSTRACT TRUNCATED AT 400 WORDS)