Quantitative enzyme immunoassays for parainfluenza type 1, 2 and 3 IgG antibodies were developed. Serum specimens were tested at a single dilution of 1:1000 and results expressed in units by the use of a standard curve. The unit values correlated well with titres obtained by testing the same specimens in serial dilutions. All serum pairs with significant titre rises also showed significant rises in unit values. Parainfluenza IgG and IgM serology was evaluated in 66 patients with a proven parainfluenza infection. Diagnostic IgG antibody increases were detected in 70, 69 and 87% of parainfluenza type 1, 2 and 3 infections, respectively. Heterologous titre rises between parainfluenza types 1 and 3 were common. IgM antibodies were detected in 42% of the patients, most commonly in those below two years of age and rarely in adults.