Development of proteoglycan (PG) induced arthritis (PGIA) in female BALB/c mice immunized with PG at different ages. Arthritis scores are indicated on the y-axis and the experimental period on the x-axis. Incidence is shown as percent in brackets next to the symbols of each age group. The first PG immunization was given on day 0, and arrows on the x-axis show the second and third PG injections, given on days 21 and 42. Animals were scored 3 times a week, and a visual score from 0 to 4 of each paw was given as described in the Methods. Number of animals in each group sacrificed on day 87 is shown in Table 1.