Methods of assaying for (i) a pterin by immunoassay employing a pterin as capture agent, (ii) neopterin by chemiluminescent microparticle immunoassay (CMIA) employing an anti-neopterin antibody (Ab) as capture agent, (iii) neopterin by an immunoassay (IA) employing an acridinium (Acr)-labeled anti-neopterin Ab as conjugate, and (iv) neopterin by an IA employing Acr-labeled neopterin as tracer; an Acr-labeled anti-neopterin Ab; a conjugate/complex comprising anti-neopterin Ab and a carrier scaffold; a conjugated pterin; a conjugate comprising an Acr-labeled pterin and a carrier scaffold; an immunogen comprising neopterin and a carrier protein; a conjugate comprising such an immunogen and an Acr compound; an immunogen comprising a carrier protein and a neopterin hapten; a conjugate comprising such an immunogen and an Acr compound; a kit for assaying a pterin comprising a pterin as a capture agent and instructions for IA; and a kit for assaying neopterin comprising an anti-neopterin Ab as a capture agent and instructions for CMIA, neopterin comprising an Acr-labeled anti-neopterin Ab as a conjugate and instructions for IA, or Acr-labeled neopterin as a tracer and instructions for IA.