Transovarian transmission of paratyphoid Salmonella is well documented and occurs at a low incidence in chickens. However, the exact mechanism of follicular invasion is not well understood. The following study investigates the ability of Salmonella to invade ovarian follicles at different stages of follicular maturity in vitro. Ovarian follicles were collected from Leghorn hens and separated into three stages of maturity: (1) large yellow follicles or F follicles (LYF), (2) small yellow follicles (SYF), and (3) small white follicles (SWF). All follicles were incubated at 37 degrees C in RPMI 1640 medium. Follicles were incubated with 1 x 10(6) CFU/mL of Salmonella typhimurium and Salmonella enteritidis sensitive to gentamicin for 2 h. Samples were then removed from the bacterial culture, and placed in medium containing gentamicin sulfate for 5 h to kill any S. typhimurium or S. enteritidis, which had not invaded the follicular membrane. After the 5 h incubation, follicles were stomached in phosphate buffered saline. Serial dilutions were made of each follicle and viable S. typhimurium and S. enteritidis cells were enumerated on brilliant green agar. Two identical trials were conducted. Data suggest that Salmonella may differentially invade ovarian follicles depending on maturity of the follicle, and that SWF may be more susceptible to S. typhimurium and S. enteritidis invasion than either the SYF or the LYF.