The levels of naturally occurring IgG and IgG subclass anti-IgE autoantibodies (a-E Ab) were studied in 71 randomly collected cord sera with ELISA using solid-phase IgE-DES myeloma protein. IgG a-E Ab were present in all cord sera, and the range was 300-fold (1.8-540 arbitrary units/ML; median = 11.8). However, this activity is the sum of two major types of a-E Ab that we refer to as isotype-specific (IS) and non-isotype-specific (NIS) because they react with E-chain-specific and myeloma-restricted epitopes, respectively. These two types of a-E Ab were distinguished in IgG subclass a-E Ab inhibition ELISA for all samples using unheated IgE-PS and heated IgE-DES as inhibitors. IS and NIS a-E Ab were found among all four IgG subclasses though with different prevalences. No significant influence of gender was observed. Comparisons within each subclass indicate that NIS a-E Ab were more common than IS a-E Ab for the IgG1 (p < 0.00005), IgG2 (p = 0.07) and IgG3 (p < 0.005) subclasses while the reverse was true for IgG4 (p < 0.00005). In fact, only a minor part of the IgG1 (7%), IgG2 (13%) and IgG3 (9%) a-E Ab activity towards IgE-DES was IS while for IgG4 it was a major part (82%). Attempts to quantify IS and NIS IgG subclass a-E Ab using chimeric IgG subclass antihapten Ab suggested that the pool of IgG a-E Ab against IgE-DES was dominated by NIS a-E Ab particularly of the IgG1 subclass. The 75 percentiles for NIS IgG1, IgG2, IgG3 and IgG4 a-E Ab were 24, < 2, 2.1 and 0.27 ng/ml, respectively, whereas the corresponding figures for IS a-E Ab were 7, < 2, < 2 and 0.90 ng/ml. The findings raise questions on the definition of a-E Ab and suggest that caution should be exercised in the interpretation of any a-E Ab results that are detected with IgE myeloma proteins. The potentially more interesting IS a-E Ab may be overshadowed by the bulk of ubiquitous NIS a-E Ab and background. Consequently, discriminatory assays are necessary if the physiological implication of naturally occurring IS IgG subclass a-E Ab is to be elucidated and these considerations are not limited to studies of cord serum.