Foreign institutional investors (FIIs) were net buyers of Rs 288.80 crore on September 21, Thursday according to data released by Sebi today.While FIIs made gross purchases of Rs 1878.70 crore, gross sales totalled Rs 1589.80 crore.Mutual funds (MFs) were net buyers of Rs 191.60 crore on Thursday. MFs made purchases of Rs 566.35 crore and sales of Rs 374.75 crore.