Objective To compare the infectivity of several transformed Chlamydia trachomatis (Ct) mouse pneumonitis (Mopn) strains to host cells, and to identify cell invasion-related virulence genes in Chlamydial plasmids. Methods Several Ct strains, including wild-type Ct Mopn strain(WT strain), plasmid-free Ct strain(CMUT3 strain), Ct Mopn strain transformed with the shuttle vector carrying pGFP and the complete C. muridarum (CM) plasmid (pGFP::CM strain)and Ct Mopn strains transformed with shuttle vectors carrying pGFP and mutant CM plasmids with in-frame deletions of Pgp3, 4, 5 or 7 (pGFP::CM△Pgp3, 4, 5, 7 strains), were cultured, amplified and collected. After the concentrations of Ct were determined, each of these strains was divided into four groups to be inoculated at a same amount(1.5 × 104 inclusion forming units(IFU)) followed by four different treatments respectively: centrifugalization +DEAE group treated with centrifugalization followed by ion-exchange chromatography on diethylaminoethyl(DEAE)-cellulose columns, centrifugalization group treated with centrifugalization only, DEAE group treated with chromatography on DEAE-cellulose columns only, control group receiving no treatment. After additional culture for 20 - 24 hours, indirect immunofluorescence assay was performed to count the number of chlamydial inclusions. At 20, 40 and 60 hours after infection, the growth rate and area of chlamydial plaques were assessed after three continuous passages. Lugol′s iodine staining was conducted to observe glycogen synthesis in bacterial inclusions. Results The inclusion number in the centrifugalization + DEAE group, centrifugalization group, DEAE group and control group was 10.20 ± 1.30, 6.80 ± 0.44, 3.00 ± 1.22 and 0.80 ± 0.45 respectively for the pGFP::CM△Pgp4 strain, 6.40 ± 0.89, 3.80 ± 0.83, 1.60 ± 0.89 and 0.60 ± 0.54 respectively for the CMUT3 strain. Under same experiment conditions, the pGFP::CM△Pgp4 strain and CMUT3 strain showed similar infectivity, and formed less inclusions compared with the other Ct strains (all P < 0.01). The number of inclusions formed by the same Ct strains were significantly different among the 4 groups(F = 845.310, P <0.01), and were highest in the centrifugalization + DEAE group for all the strains. The pGFP::CM△Pgp4 strain showed significantly lower growth rate and area of plaques with an abnormality in glycogen accumulation compared with the other strains at 20, 40 and 60 hours after infection. Conclusion The plasmid-encoding gene Pgp4 may be a cell invasion-associated virulence gene in chlamydial plasmids.