The effects of human rheumatoid arthritis [RA] sera on PMN [polymorphonuclear cell] functions were investigated. Results revealed significant inhibition of phagocytic activity (PA) and nitroblue tetrazolium (NBT) dye reduction capacity of cells from RA and from a normal (N) control group. The number of PMN showing aggregated latex particles on their surfaces was significantly higher in RA patient groups in the presence of rheumatoid sera. PMN from N and RA groups were lysed in the presence of rheumatoid sera, with more significant destruction of RA-PMN. No correlation was found between PMN function and the amount of circulating immune complexes or rheumatoid factor. Significantly lower concentration of complement C4 levels and higher IgA mean values were observed in the patient group compared to the standard values. Mechanism(s) underlying the impaired PA and NBT reduction may be due to defective intrinsic cell function and the extra cellular effects of blocking or lytic factors present in rheumatoid sera.