A simple, rapid screening method using alizarin red S stain and ordinary light microscopy to detect microcrystalline or noncrystalline calcium phosphate salts was used on wet drop preparations of synovial fluids. This proved to be helpful in detecting apatite crystal clumps and small calcium pyrophosphate dihydrate (CPPD) crystals missed by polarized light. The staining was positive in 100% of synovial fluids from patients later proven to have apatite and/or CPPD deposition diseases. Apatite and CPPD crystals were commonly found together in the same fluids. In addition, some synovial fluids from patients with osteoarthritis, renal failure dialysis, rheumatoid arthritis, and gout also exhibited positive staining. The correlation of positive alizarin red S staining with radiologic evidence of osteoarthritis suggests that apatite crystals might be related to articular cartilage degeneration in different rheumatic diseases.